You will need to consider the siding and roofing materials you would like to use to finish this space. And you might weigh the quality of exterior materials against their visibility. Also in the cost of the addition, be careful to factor in the permits needed and the work needed to prepare the foundation.
Envelope size - 4-3/8 x 5-3/4. Card paper size - 5-1/2 x 8-1/2 (half a letter sized sheet). Finished card size - 4-1/4 x 5-1/2. A4: Envelope size - 4-3/4 x 6-1/4. Card Paper Size 8 x 12-1/4. Card finished size - 4-1/2 x 6-1/8.
For most ID cards and credit cards, ID-1 from the ISO/IEC 7810 (Wikipedia) standard gives the dimensions, which are 85.60 Ã 53.98 mm (3.370 Ã 2.125 in) with an aspect ratio of 1.586. This is also the standard business card size in many situations as it is highly pocketable. Another size which could be considered somewhat international, is the ISO 216-sized (Wikipedia) A8 = 74 x 52 mm (2.913 Ã 2.047 in) with an aspect ratio of 1.423.