I did the vellum idea from above (with the picture on cardstock, vellum invitation on top of it) for our wedding invitations. Well, we never used them. (another story) Anyway, I did quarter pages (ended up about 4x6 size - perfect to frame one for your MIL to keep). They were still very pretty and I didn't think they were too small. For 300 people, you won't be sending out 300 invitations unless they only know single people.

So figure out how many invitations you need, divide by 4 and let Kinko's do the work. They'll even cut them for you if you want.
Another idea is to make a digital scrapbook page of pictures, invitation wording, etc, and then have them printed as photos. 4x6 or 5x7 you wouldn't have to do the traditional "you are cordially invited to..." wording, but you could do something more like
______ & ______
A Union of Everlasting Love
July #, 2007
And then on a small business card have the details of where, when, reception details, etc. Just another thought...