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.
The term A3 refers to a specific size for paper. An A3 sheet of paper measures 297 x 420 millimeters (11.69 x 16.54 inches.) This size is standardized by the ISO paper size system.