I hadn't realised, until I saw it mentioned at the LB Yahoo! group, that in LB 4.04 and 4.5.0 the RESIZEHANDLER command doesn't work in a graphics window. This seems a very strange limitation (not the first!) and it would never have occurred to me to do the same.
So, in LBB, you can use RESIZEHANDLER with a graphics window just as you can with every other (non-dialog) kind of window:
open "Resize test" for graphics_nsb as #w
#w "trapclose [quit]"
#w "resizehandler [check]"
print "Width = "; WindowWidth; ", height = ";WindowHeight