Wouldn't 5" x 7" frames work? They would have a bit of clear border where the wall would show through.

If they have a white edge all around, you could use the 5x7 and put a piece of white paper behind the postcard to fill out the frames. It will just look like a slightly larger white border on the postcard (once they are squished behind the glass).

i think i would drive myself batty centering them in a larger frame? and how to make them stay?

If you want to use glass, can't you get bigger frames and have the glass cut down? Or just go to your frame store and have them sell you the right glass and clips

Why not have then professionally framed? 42 years ago while in university, I bought some darling panda large postcard prints at the national zoo and my Mother had them professionally double matted and framed for me. They lived in my college apartment and later my house house through my 20s and later found a home in one of DH's operatories. They still looked great when he sold his practice last summer. Sure, the framing was expensive compared to the price of the prints, but after 42 years of use, well worth the cost.

I just ordered from a company that will make you any size custom frame that you want - www.theframeguys.us. I think they are also on etsy under the seller name Rusty Roof. I have also ordered custom size frames from framesbymail in the past but their styles were more limited....theframeguys had the black narrow bamboo frame molding that I had been running all over the place trying to find. At any rate, I called them to order and they are the nicest family-run business. There was an issue with my order originally (a mix up in how I thought I was giving them the measurements and what she thought I was giving her) and they took total responsibility and replaced the frames. The cost for 2 frames was less than what I would have paid to have 1 custom framed at AC Moore and AC Moore didn't have the color I wanted. I'd definitely recommend them. Just be sure you tell them exactly how you want the art to fit in the frame - there is an overlap where the front of the frame molding covers the glass and some frames want you to take that into account when measuring and some don't.

Thanks everyone for your input. I think I will just have to go the 5x7 route (thx for the mounting tape suggestion). If my DD is anything like me, her taste in art is very mercurial so to literally frame them is bad value. Plus, I really like the glass frames because they disappear, which is what I want. I just have to see how it looks with the wall color showing thru. I would go custom but they are too cheap for anyone to make custom, and it is not just cutting glass; the back is usually fiberboard. Oh wait, hmm, then that won't work either. Darn museum, what is wrong with good old 4x6!

A glue stick is even less noticeable (& easier to remove) than mounting tape & is also available in archival formulations.

Hmm, wonder if you could have them enlarged to the 4x6 size at a print/copy shop? Some places may give you a copyright problem, others do not care. Good luck.