I was having the same issue myself.
So I poked around in the plugin files, found a solution & thought I'd share.
NOTE: This type of Trial+Error also works best if you have a little separate testing-area install of WP on a subdirectory of your server like /newfeaturetests/ or whatever.
-So you don't screw an important live-site horking around with your core php files, by making some little syntactical or punctuation mistake.
Just fill it with dummy headers, entries, images, etc. and fool around with no/low consequences.
If you go into Plugins>Editor, and look in the files:
On Line 510 of the OpenGraph one you should see:
Change medium to the image size that you would like. In mine, I just put full, because mine aren't that big.
Also, Put a note in the nearest comment area of the php file that you changed something to remind yourself what, when & where.
Save the changes with "Update File".
In the file for the Twitter Cards its on Line 137.
I haven't looked in the Schema one, but a search for the word, 'medium' and then a change to your desired size should give similar results. (upd: It might be the one on Line 410)
Do the same thing, super-reload the page & see what happens.
Best of Luck; Cheers.