What type of tree? The size may be determined by size of restricted rootball in a pot but they need a lot of water in pots.

Yea, which type of tree - seems to be something missing from your question, you've just said 'as' - ash? acer?

Some trees do better in pots then others. Some conifers only max out at 8-10 feet. These would be comfortable in a tub given sufficient water and food. The word "reasonable" is relative, but you can usually maintain desired size by trimming the main leader. Everything really depends on the cultivar, environment and if you are willing & capable on giving it the proper care. As alluded to above, potted trees are very hungry & thirsty and are 100% dependent upon you.

On the other hand if you really mean an ash tree it will get much too big for a pot whatever you do unless you bonsai it.

I have an ash tree kept in a pot as a bonsai (dwarf tree) which is easily done: keep in well-drained gritty compost and prune the roots every Spring. Only prune the branches to keep a nice shape. Keep it watered in the growing season.

I hope Russty puts up another comment to confirm what sort of tree it is....