I use a

28"(27.5 viewable) Hanns-G HG281D monitor with a resolution of 1920x1200

and

22" HP w2207 monitor with a resolution of 1680x1050

I had no trouble with isboxer switching, not sure what the PIP feature of isboxer is, I just had master on main monitor and slaves on other monitor. Just have to set it up in isboxer monitor manger.

However, I believe the fast switching of isboxer is more dependent on how "good" your system is in terms of speed and stability rather then what monitor(s) you use. Keep in mind that the more monitors and the size of the viewing area and higher resolution will put a strain on your computer. So a "good" computer will have no trouble with the fast switching of isboxer.