I am puzzled how or why the shower is to be "downsized." How small do you want it to be? What will go in the reclaimed space? Do you want to make it shorter or more narrow? What size is it now? I find that 36 inches wide is comfortable, and 30 inches (minimum width) is awkward.

How much smaller do you want the shower to be? What about adding a bench to take up some of the space.

It’s 52 inches wide. Possibly adding built in shelves along left sife?

Maybe going to a 36 inch wide would be nice, not looking for a small shower. This one is just so big!

This looks smaller. You'd need to deal with adding walls.