There is no one standard size for wedding invitations. The size largely depends on the standard sizing for envelopes as well as the availability of those envelopes. You can create invitations on any type of paper, but they generally fit within one of three standard announcement-sized envelopes.
Standard envelopes with the designation of "A" are considered announcement envelopes. These envelopes are commonly used for greeting cards, announcements and invitations.
Announcement envelopes typically have a V-shaped pointed edge that is glued down or a square-shaped opening edge.
A-7 sized envelopes hold invitations that are approximately 5 inches by 7 inches. The envelopes are 5.38 inches by 7.63 inches on the outside and 5.25 inches by 7.25 inches on the inside.
A-10 envelopes hold invitations that are 4 inches by 9 inches. The inner dimensions of the envelope are 4.13 inches by 9.5 inches and the outer dimensions are 4.38 inches by 9.75 inches.
A-2 envelopes hold invitations that are 4.1 inches by 5.25 inches. The outer dimensions of the envelope are 4.375 inches by 5.75 inches, with the inner dimensions only slightly larger than the recommended invitation size.