The best thing is to put the pots in the ground. You can't keep them outside in pots, they will get wet, and the roots will be frozen and they will die. The garage is better. If possible put them on their sides to stop any growth, water about once a month. You don't want the soil to completely dry out. But you don't want it very wet either.
I never dealt with pine trees, but this is what i would do with fruit trees. Cover the roots with hay or blankets if possible too. They need to be insulated, but even if not and even if upright, should be ok.
Plant them ASAP in the spring. Don't worry about frosts, as long as the ground is workable, you can plant them. Last spring our last frost was April 25th, but I planted trees March 15th, the ground was no longer frozen. The trees did great!

I'm wondering what planting zone you are in and why they would send you trees in Dec.? Doesn't seem like a good plan.

A lot of the west coast Nurseries offer their bare roots in December. Latest for CA is the first week in March., I have ordered bare root from Bay Laurel, and March, the first week is the very latest they can send bare root. Grow Organic can send at the latest January.
I like to experiment with some west coast trees and bushes. Best to buy close by, but many new fruit tree hybrids are developed on the west coast, and I want to see how well they do here. I have some Southern Highbush blueberry bushes coming in March from Bay Laurel.