Hi bartbart!

I don't know about root pruning, but about pot sizes, the rule of thumb I always heard is that the pot height/plant height ratio should be about 1/3, and 1/4 at the barest minimum.

^^^ The above is really only an aesthetic consideration and has nothing to do with containerized plant health.

I grow a whole collection of trees in containers and every year there is a group that requires root pruning and repotting :-) Generally, this is a midwinter project as you want the trees to be as dormant as possible. I remove the tree from the pot, slice off the bottom 2-3 inches of the rootball and shave down the sides. How far I shave depends on how congested the root system seems but it is rare that it is more than a half inch to an inch.

I wash the roots of all soil as best I can and then do any further root trimming as necessary, typically just to make sure cuts are sharp and clean and the roots not torn. I use an old pruning saw for most of this but finish off with just sharp pruners. I then repot with fresh media.

How much root pruning I do when repotting also depends on the pot size. Many of my trees are in pretty generously sized pots and are as large as I wish them to get. Since I am NOT increasing pot size, I root prune harder than I would if I was just up-potting a small tree to a larger container. In some cases, I do not root prune these at all at repotting.

The tree in the photo looks like a dwarf Alberta spruce. These are small y habit and very slow growing and with regular periodic root pruning, you should be able to keep in that sized container indefinitely. Container gardening is a natural dwarfing or stunting process anyway........think bonsai (although that is taking it to the extreme :-))

Thank you both so much. I hope to get my tree to grow taller; as it is now, it is short and pretty fat; actually probably propotionally a little less tall than it's pot. But it dries out WAY too fast,so I'm going to do the root-prune as gardengal describes.