I highly recommend their new widescreen tablet. I’ve got one myself.
Getting a regular 4x3 tablet will either mean wasted space on the tablet that you won’t use, or distorted dimensions, where drawing a circle on the tablet will result in an oval onscreen, depending on how it’s set up. The widescreen is probably your best bet. With a grip like you use, I doubt the 4x5 will be big enough for you, and the 9x12 is only an inch wider than their 6x11 widescreen, but with three extra inches of height that would just be taking up space. My 6x11, coupled with a 24" Dell 2405FPW is perfect.
Your grip might cause some issues, though. One of the many things a Wacom tablet keeps track of is the angle of the pen against the surface. Airbrushes in Painter will spray off in one direction if you hold the pen at an angle, for example; though you could probably turn that feature off if it became a problem. I doubt the pen will work very well at a VERY large angle, though.

