There are some long standing beliefs that tablets must get bigger as the monitor gets bigger. In some cases and for some users that can be true. I've never found there to be much relationship between the two for me.
One of the biggest factors that should determine how big of a tablet you need is how you use it. Do you mostly make fine motions with your fingers, or move your wrist, or make broader sweeping strokes with more of your arm? Obviously the bigger the stroke you want to use the larger you might want your tablet to be.
The first two tablets I had were similar in size, and I've had tablets since the late 90s. During that time screens got bigger and bigger, in fact my monitors are the biggest they've ever been and newest tablet is the smallest I've owned yet.
So, I don't believe we can really tell you which tablet is the right size for you. You'll have to figure that out for yourself based on how you are most comfortable interacting with it and what you need from it.