This question got me curious, so I started googling.
Keyboard size is not officially standardized (there is no committee creating and enforcing standards), but in practice, there is very little variation.
Browsing through forum topics on www.pianoworld.com, people measured 88 key keyboards from anywhere between 48 inches to 48 1/2".
Wikipedia ( http://en.wikipedia.org/wiki/Musical_keyboard ) describes the width of an octave on a modern keyboard as 164-165 mm, which leaves almost a centimeter variation in width over the entire keyboard.
There is also the option of buying a deliberately manufactured smaller keyboard, as described at http://www.smallpianokeyboards.org/keyboard-history.html .
This would have a width as small as 141 mm for an octave, or over 2 cm smaller than a standard.
I don't play piano, but as a small handed woman, that sounds pretty attractive.
Historically, from both the wikipedia and the smallpianokeyboards.org sites, older keyed instruments had octave widths as small as 125 mm, but not modern pianos.
For piano-like instruments, such as midi-controllers and cheap electronic keyboards, smaller keys are common when manufacturers have goals other than producing a standard piano.
music.stackexchange.com/users/10353/Karen
While I see no reason to dispute the answers already presented I thought I would follow up with posting the measurements that I proposed in my comment under the question.
Measuring several keys near middle C with a calibrated digital caliper: 1915 Steinway Model M (New York) with original action: 22.1 to 22.
9 mm (slight variance),
+/-
fractions of 0.86 inches 2002
Yamaha Model S08 88-note weighted-action synthesizer: 22.8 mm or 0.89 inches
I suspect the variance of the width on the Steinway is due to the 1915 tooling vs. computer assisted manufacturing of the 2002 Yamaha.
To get a more authoritative answer I have contacted an authorized Steinway tech that tunes my Steinway and restores them as well (wish I could afford that) to review this.
Pending his email answer, I will post an update later.
UPDATE June 30th, 2014:
It has come to my attention that the following answer would be more valuable to the community if I were to credit my source properly.
I have been reluctant to reveal my source these past few days as I did not want him to be subjected to unnecessary distractions (spam) or harassment from adversarial points of view.
As I stated clearly last week "protect my source.
"
I have been in contact and we have reviewed our options on this post.
So without any further chit chat, I would like to introduce to you, John Callahan of Callahan Piano in the San Francisco Bay Area, a Steinway restoration specialist.
http://www.callahanpiano.com/
Please be nice to him, he gets it right every day from tuning to restoration and beyond.
"You are correct, 100 year old piano keys will vary slightly in width.
Some of that is the original tooling and cutting (piano keys are sliced like bread from one piece of material) and some of the variance can be from wear.
The keys at the far ends of the keyboard will be a good bit wider on an older, heavily played instrument than the keys in the middle of the keyboard, where the most playing occurs.
" "No standard width.
You can measure many different pianos, and while they will all be within a fairly standard range, you will see differences.
Same goes for the sharps.
This also holds true for many other piano dimensions.
Keytop thickness, head length (distance from the player's end of the natural key to the cutout for the sharp), tail length, height of the keytop off the floor, height of the pedals off the floor, etc."
music.stackexchange.com/users/2076/filzilla
Do other keyboard instruments have significantly different widths of keys?
Organs seem to have the same key widths
but I can't verify that.
Accordions do vary - I can just reach a 10th on a piano but can easily get at least a 12th on an accordion.
music.stackexchange.com/users/12190/ouchjars
7/8" appears to be the industry standard, but there is a slight (2 or 3%) difference occasionally.
Some keyboards I think are slightly smaller, but can't access mine for a while, so can't measure.
I'm not talking mini-boards here,which are made either for kiddies or for studio pro. use.
Got a Yamaha keyboard here with 7/8" keys.
I think the main differences, which subtly affect things, are the length of keys and the amount that they get pressed down.
That, and the different 'balance' feel, will make each piano different, but width-wise, not a lot.
music.stackexchange.com/users/2808/Tim
Do other keyboard instruments have significantly different widths of keys?
I can speak from experience that melodica keys are significantly narrower than piano keys.
Where I cannot comfortably reach more than an octave on a piano, I think my range is more like a 10th on the melodica.
It was surprisingly tricky to reprogram my fingers for the shorter scale.
I can only imagine that an accordion would have a similarly-shortened keyboard.
In addition, Karen mentioned smaller keys on some midi controllers.
Here is an example of one such device, designed for portability.
music.stackexchange.com/users/10637/Caleb Hines
My Evolution e-Keys mini USB piano keyboard has 17 mm wide naturals with 1 mm gaps between them so the center-to-center measurement is 18 mm, 5 mm less than what is normal on a piano.
The distance from the front edge of the naturals to the accidentals are 30 mm while on a piano this is 50 mm and on organs often 46 mm.
music.stackexchange.com/users/21120/Magnus Johansson
Is there a standard width for piano keys?  If not, how much variation is there?  Do other keyboard instruments have significantly different widths of keys?
Is there a standard width for piano keys?
music.stackexchange.com
<piano><keyboard>
music.stackexchange.com/questions/20290
music.stackexchange.com/users/9198/Bob Broadley
Keyboard size is not officially standardized (there is no committee creating and enforcing standards), but in practice, there is very little variation. 7/8" appears to be the industry standard, but there is a slight (2 or 3%) difference occasionally. melodica  keys are significantly narrower than piano keys. There is no official enforced standard but there is likely to be little variation, with a common measurement being around 7/8". With regard to other keyboard instruments, the melodica has signficiantly narrower keys than the piano.
There is no official enforced standard but there is likely to be little variation, with a common measurement being around 7/8". Melodica keys have significantly narrower keys than pianos.