What makes things more confusing is that the images of the posters that are printed on the front of the cards look a bit different than the posters that are on display with the Harry Potter Exhibition.
Let me know what you think!
I think that these are wizarding posters, and the images change constantly.

I'm watching again once I read this thread, so far through the approach to Borgin & Burke. There's a Fenrir poster in the foreground when the trio enters Nocturne Alley, but you can't tell the size from the perspective. All three of Fenrir's shots are moving, but the left one is primarily for the left profile and right for the right, the opposite from the Exhibition pix. But the Amycus and Bellatrix Exhibition pix look the same as the pix on the prop cards, so there could still be a size difference in the swatches.
A slight bit later the trio is standing next to three posters. They're out of focus, but the left one seems to be Alecto. The posters are a good 2-1/2 to 3 feet tall. I can find logic in different sizes of posters for windows versus walls, so if they're sometimes smaller, maybe it's like the "mini-posters" Topps uses as box toppers. We see 8-1/2 x 11 "posters" at business places, which are almost worth calling "handbills." Just before the visit to B&B you see some papers blowing down the street, but I can't say they're posters.
As for writing on the posters, check this fine variant out (and it has a very nice super- and super-size image still):
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=120508465748. (Thanks, Adamxxx!) I don't have a clue what the numbers stand for, but the small ones are one pica wide (10/inch), which would make them about 14-point type with most modern typefaces, and the "1" is larger. If I guess that they come from the two lines just below "Reward" on the poster, I could convince myself that the whole poster ratioed to about 10-12 inches tall. If they come from the tiny stuff at the bottom, the original item could be bigger.
(Oh, and for your close-up of P10, I didn't see a face on Mars or a potato chip shaped like Elvis either. [ducking] I could buy the possibility of a small face in your Alecto poster, but not after I saw my numbers. If that swatch is from the collar/neck in the right mugshot, the whole poster might indeed be 2-3 feet tall.)