Yes, you could keep a ceanothus pruned to nearly two dimensions (width and height and not very deep).

I think your problem will be the 5 x 5 foot dimensions.
In a very few years a ceanothus will have to be heavily pruned to stay that small and may end up being more wood than foliage.

That plant blooms at about the right time for Tiger Swallowtails (mid-June to mid-July), otherwise you will have mostly bees.

We have a ceanothus hedge at a commercial building that has been kept pruned to a 3 foot tall height for over 15 years. It is in the middle of a road/blacktopped parking area and gets little attention, including water. We had frost damage one winter but it has grown back and still looks very acceptable.

Ceanothus is commonly used for hedging material in this area, usually in commercial situations where attention is minimal other than routine pruning to keep to size (3' is pretty standard). Not only does it seem to thrive, it blooms nicely as well.

Or you can select the variety 'Vandenberg', which is a quite compact cultivar that will stay very close to the desired dimensions without excessive pruning.

Cut back every year, right after bloom, stay out of the old hard, less leafy or leafless wood.