Hi,
In case your plate reverb project is still active I can give you a contact that sells parts, i.e. the driver and pickups + has a lot of good info.
I am doing a plate reverb myself as a slow side project and recently ordered the parts. Haven't got them yet though so I won't guarantee nothing.
Regarding plate size, the smaller it goes, the thinner it must be and the thinner it gets, the harder it is to stretch it without damaging it, and the lesser the stretch, the uglier the bottom response.
Smaller plates would also have lesser bottom response, but that doesn't need to be a bad thing if it sounds smooth.
Recommended thickness is from 0.4mm to 0.9 i believe.
Another note:
Cold rolled steel --> Smooth hall like sound
Stainless steel --> More metallic sizzle
So in other words, if you want to downsize you should try cold rolled steel as thin as you can get it. Sheet metal is pretty cheap so you can experiment in that department.
Good luck!
hejsan