Google the 50A55-486 and see if it definitely says it is a replacement for your board. That sounds like a White Rogers number and they do make many control boards. If it's a direct replacement, download the instalation manual and read it comparing their connections with the connections on your old board. The connection terminals should be similarly labeled. If you can understand how to wire the new board, then you can get it.
As for size, that's not important provided you can fine one or two screws to fasten it securely to the furnace. You don't want the bare PCB to be able to contact the metal furnace frame. But it doesn't have to look pretty, it only has to function.