The size of the initial pot should be considered a judgment call. Your root-pruning efforts last fall should have paid off in the form of more fine roots close to the tree, but how many fine roots there are sort of dictated how much you can prune off the roots to fut it into a pot - especially since you indicate the plant was stressed going into dormancy. It didn't GAIN any energy over winter, so it will be weak now, and needing it's carb reserves.

The soil should be free draining and well aerated. I'm going to suggest you read the link below, which addresses container media. The recipe called 'the gritty mix' at the end incorporates almost all (if not all) the desirable characteristics of a bonsai soil, and it's adjustable for water retention. If you have more specific questions, you can ask there or here.

Repot at the first sign of bud movement. After that, you'll need to protect the tree from freezing temps, so moving it in and out of the garage or shed after it leafs out, until temps are reliably above freezing, is the best plan.

The larger the container and the more air the soil holds - the better, but if you choose a heavy, water-retentive soil like MG or others based on peat, container size is much more critical. After you make your soil choice, I can help you with tips on how to get the most out of whatever you're using - and I can help with a fertilizer supplementation regimen.

The basic soil I use for all my trees
{{gwi:1295}}

Al

Here is a link that might be useful: About container soils in general - a concept