I've been doing some fiddling around with this today to see if I can get 6 windows (main + 5 alts) up on a 1920x1200 resolution, essentially a layout like this...
After a fair amount of testing, and experiencing the same issue as you, it would appear that there's a minimum window size below which the client just exits. I believe this to be 800x600 based on the config tool. Kind of a pain, though maybe the excuse I need to buy a new monitor?