Any measurement you like: volume of air, duration I can hold my breath, and most relevantly, how quickly I get "winded".

There are specific swimming exercises you can do, as part of a routine, that build lung capacity.

(Results may not generalize. It worked for me.)

I imagine that anything that taxes lung capacity will help. I’ve picked up the French Horn in quarantine, and it seems to be strengthening several mouth and jaw muscles, along with the improvements to lung capacity you’d expect.