OpenEuphoria

I'm not sure what exactly you are asking for but I'm guessing you want Program-A to get the size of a window that happened to be created by Program-B.

If are developing both programs, you could set up a private API such that Program-A can send a message to Program-B requesting the information. Do some research on Inter-Program Communication for this approach.

If you have no control over Program-B, then you need to use the Windows API to get the information once you have made Program-A find the handle to the window. This requires that Program-A knows what the window Title would be ... would that be known to Program-A?

By the way, what are you planning to do with the size information once you get it?