It is probably 2 to 3 gallon, just my favorite size. Not too big to have to work in stages to cut it down or so small------you have to work in stages to grow it up. Good workable Pre-Bonsai size. It's small enough you can find a solution to make it into a shohin. It's large enough you can find a solution today that will turn into a good tree in a couple of seasons.
As to repotting the tree. If this was mine I would do what I usually do. I would reduce the soil ball down by 50% right after Father's day, late June. Do not remove the 50% by digging it out and cutting it off with scissors. Find a good sharp saw and cut it off. Quick and clean with not additional stress on the root system. You can also remove the upper branches as long as you leave a stub at the base where, the one branch you are keeping emerges, assuming that you are going to keep my design idea. Anything you want to do is OK but just remember; think small and succeed large.
Comb out the circling roots growing around the soil ball just enough to free them up, do not dig into what is left of the old soil ball. I would then plant the tree in a colander with good quality bonsai soil to give it a chance to develop some fine feeder roots in abundance so that the soil ball can be reduced again enough to put the tree in an acceptable bonsai pot. During its time in the colander you can prune and wire the tree and start to establish its shape.
Remember one important thing. Once you cut it every body will recognize that you have a good tree worth the effort to make it into a good bonsai. Every body will be coming out of the wood work making suggestions about what to do and when to do it. The important thing is; this is not a Japanese Black Pine and can not be handled in the same way many who grow Japanese Black Pine are accustomed to do, and those who still believe that working with all two needle Pines is the same as working with JBP's. I have said it before that more trees are killed by kindness than by neglect. Do not submit to the kindness of those who will undoubtedly tell you what you are doing is wrong, sadly most of those individuals do not have a Mugo Pine long term.