I agree with the 4 ft net for what you are planning. A smaller net is cheaper for one thing and you can throw it farther if bait is further out from the rocks. You can catch all the bait you need with that size. Go on youtube. Everybody and their dog has put up their way of casting a cast net. The simpler methods mainly rely on centrifugal force to twirl the net and will work better with a smaller size net.
+1
I bought my first cast net a month ago (5ft)
I tried throwing it with out youtube first and kept throwing taco's.
Watched several different methods on Youtube and practiced in the yard and caught my first shad this past weekend.
it was very fulfilling to catch your own bait.
Good Luck!