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The largest letter that can be mailed with a regular stamp is 11 1/2" X 6 1/8." Further info can be found on the official USPS page at: https://www.usps.com/send/can-you-mail-it.htm?#3
For a standard size envelope it is 44 cents.
3 1/2" x 5" (inches) is the smallest legal mailing size for an envelope in the US.
It is not the size of the envelope that matters, it is the weight - it is probably best to have the package weighed at the Post Office to obtain the correct postage.
3.5 inches by 5 inches I mailed one out and it seemed to work. Anything smaller will be kept by the post office or returned to sender.
US Mail standard envelope up to 1 ounce in weight needs a 42 cent stamp (or any combination of stamps that total 42 cents.)
One normal Canadian stamp (for a letter size and weight)
one ounce.
It depends on the size, weight, number and destination
11 1/2 inches by 6 1/8 inches
One 49-cent stamp should be all that is needed.
It depends on what country you are posting from, where to and, possibly, the weight of the package.