I have a few trees that seem very large for the size pot (3 gallon) they are in. They rooted into the ground mid summer and I left them. Now they are dormant. I cut the roots and I have them in a detached unheated garage now, but I'm wondering if they would do better in a larger pot. I'm sure the pot they are in is all roots. My thinking would be if I put them in a larger pot now they will have a little more insulation when it gets colder and retain moisture a little better. Or should I just wait until early spring as usual?

As long as the soil is well aerated it should be fine either way.

If they are already dormant and root bound, it would do no harm to transplant them now so that when spring time comes they are already settled in and ready to break dormancy.

Roots will continue to grow down to about 45 F.
So if you put in bigger post now,  they would probably establish roots into the new soil and be more ready for spring growth.

Yeah, the mix should be ok. I'm just a bit nervous. Last year was not a good test of storing my trees in the garage. It was a very mild winter. I can always try a few and see. The only thing If I do it now I won't mix in the slow release fert that I would if it were spring.

Don't forget to untangle the root ball. Perfect time to do it.

Yeah, plus the more I'm thinking about it maybe its not a bad idea to start now. I have so many more trees to do then last year.