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.
There are a lot of mathematical fomulas behind the workings of A, B and C paper sizes but the underlying feature is that any successive paper size (eg. A1, A2, A3, A4) measurement is determined by halving the dimensions of the preceding one. For example the most commonly used paper size is A4 (297mm x 210mm) and the next paper size is A5 (210mm x 148.5mm) which is equal to half of the A4 dimensions. The main uses of A, B and C paper sizes is their application to a particular print project. For example C paper sizes are solely used for the manufacture of envelopes and are sized in a way that would allow an A4 piece of paper to fit nicely into a C4 envelope.