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.
If you use an A6 paper, which is actually an A4 paper that is folded twice, then the matching envelope format should be a C6 envelope. A C6 paper format should be matched with a B6 size envelope (125 mm x 176 mm) while a C5 paper should be matched with a B5 size envelope (176 mm x 250 mm). A C4 size paper should be matched with a B4 format envelope (250 mm x 353 mm). Finally, a B4 size paper should be matched with an E3 size envelope (280 mm x 400 mm). There is a different standard that is in use in North America. However, it should be noted that not all envelope sizes are used for sending mail.