Technically, higher main magnetic field strength doesn't mean more resolution. There are three parts to the scanner: 1) main magnetic field, 2) gradient coils and 3) RF coils.
The main magnetic field strength (1.5T, 3T, 7T, 9T, the last two typically more for research) determine more of the signal to noise (in a very simple sense). This is static - it doesn't ever change.
The gradient coils are what determines the resolution and the rate of switching is the biggest determining factor. There are limits on what they can switch at when used on humans as they can cause peripheral nerve stimulation, vertigo, nausea, metallic tastes etc.
I don't believe static field of 20T etc would be a problem, though there is not much research to prove or disprove that. The problem would be moving into that field as that will cause the same types of effects as the gradient coils switching. Even for 7T, one must not run into the scan room as it can cause peripheral nerve stimulation (and some other effects). Currently, I believe the strongest full body human MRI scanner is 10.5T. Another article talks about 11.7T human scanners (one of them is a "head only" scanner. It also has some nice pictures).