All I have is a dumbbell, but I'm looking for the fastest way to build size (strength can come later) with what I have available to me.

I currently do 12 reps on one arm without stopping and then doing the same on the other arm using just one dumbbell. Is that fine or is it better to have two dumbbells and alternate sides after each rep?

All I have is a dumbbell, but I'm looking for the fastest way to build size (strength can come later) with what I have available to me.

and what muscles does the "dumbbell shoulders press" exercise benefits?