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.
Dimensions of the A series paper sizes 4A0, 2A0, A0, A1, A2, A3, A4, A5, A6, A7, A8, A9 and A10 in both inches and mm, cm measurements can be obtained from the mm values and feet from the inch values.