You say "I want to insert value returned by the function", yet your code isn't doing anything with the value returned by your function.    You need to write out a list of steps that you want to perform, then look at how to code those steps.   From your example it's not obvious what you're trying to do.

The global function is returning the form name that is calling it (it's being called from an On Click event of a button)--that part is working. And to check that, I message boxed the string variable that it's returning. The first message box is only to verify that it's returning the correct string value. In the second message box, the form name is "hard coded" in and it does return the window height. What I am trying to do--is to replace the form "name" where it says Forms! [here is where the string variable goes that contains the form name].WindowHeight. I've tried this: Forms! & ThisFormSize & .WindowHeight.  I've tried: Forms! & ""ThisFormSize"" & .WindowHeight. And lots of other variations with ampersands and quotes. I can't get the syntax correct, I guess. What is the correct syntax?

Try this:

Forms(ThisFormSize).WindowHeight

Yes, that works. Now I see that using the parenthesis is like the syntax of the CurrentProject.Allforms(ThisFormName).Name

But I never would have thought of it. Thanks so much.