The window that pops-up is scalable, see image1 of the mouse cursor to drag the window size, not to mention you have the scroll bar/s that you could use.
Personally I don’t use the forum spell checker, but an add-on for firefox (image2), most browsers have options to include a spell checker. The advantage of the firefox add-on is that highlight spelling errors in place (image1) allowing you to correct or select the correct spelling.
Even though I don’t use IE8 as my default I have opened this topic and run the forum spell checker and it doesn’t resize for me either, but that surely is an issue with IE8 and not the forum software as it is IE8 that controls the window, otherwise I wouldn’t be able to resize the firefox window.
However, the scroll bar works as does the mouse centre scroll wheel, so surely you aren’t saying that they don’t work either. Also the zoom out also works to show everything within the window including the buttons.
No one said you are, I simply quoted my experience of being able to resize the window (so not a forum software issue) and at that point you hadn’t said what browser you were using.
Just giving you options, your choice to use it or not.