My husband and I live in an apartment, so we are restricted to container gardening for the time being. Our tree will be outside year-round (our kitty thinks the ornaments are her toys). The trees that I have seen in the shops around here are about 2-3 feet (60-90 cm) tall, and are some variety of fir.

I've no idea how fast they grow so I have no idea what sized pot to plant it in to give it room to grow. We'd like to eventually plant this tree in a yard.

Thank you!

At least 5 gal. maybe even 7 gal. Also think about buying (or building) cart to put under the pot so you can move it around on wheels. Don't forget to water it every week. and feed it every three months. Trees in pots eat up the nutrients in potting soil every fast. Every year you need to pull the tree out of the pot and clip the roots that are surrounding the pot. It's very easy but messy. Clip all the roots that are turning and brush them out of the ball. I crush nut shells (pecan - English walnuts - and such) and add them to the soil of every tree I plant. (In pots or not) In pots I add a few to the top of the soil and break the top of the soil and mix it in. There is no better "slow release nitrogen" in the world.
You will still need to feed the tree every three months with a water additive such as Miracle Grow or Peters Plant food. Good Luck and enjoy your tree. I hope you plant it in the front yard of your Dream Home.

All of the pine trees I have in my yard started out as Christmas trees. Two of them started in 5 gallon containers which is adequate for a 2-3 foot tree. The varieties I had were slow growing and although they were root bound by the time I put them in the ground, they have grown to be two stories tall in 10 years. Depending on how long you keep it in a container, you may have to transplant it eventually into a 10 gallon container.