You are considering OAK for a tree that does "not get too big" ...? The average oak here is live oak and it is bigger than just about any other tree. Please show a picture of the size limiting factors. How big, actually, is the space?

Its about 17 feet wide

Is it full sun? If so:

I love my white Geiger (aka Texas Olive). It flowers and will not become big. For easy you can also use a lugustrum. They require some pruning to please some folks taste although I have seen some left natural / minimally pruned that were lovely. Also certain magnolias. Little Gem is a nice, smaller variety. Crepe myrtles are nice too.

A lot depends on if you want evergreen, flowering, etc.

A lot depends on your needs and wants and the information about that is scant. You might check around your town or neighborhood and see what kind of tree would fill the bill and then try to get it I.D.'d.

In addition to the aforementioned trees, a Loquat might be a possibility.

here's a recent thread about blocking view of air conditioning units that might be of interest to you:

Here is a link that might be useful: Blocking air conditioners