You came to the right place. There will be lots of advice given here!
I can give you my sequence: 19' 40hp O/B, to 21' Formula 350 small block, to 24' Formula small block, to 33' Baja twin big block, to 42 Fountain twin 525. From 20' you could easily go to a 23-24' single BB and if happy with the handling and performance move up from there. Cobalt, Sea Ray, and Baja all make good semi-performance models in that size range, with Baja also making several true entry level performance models. We loved our 33' Baja and kept it for 3 years. But it would not have been an easy move from the 19 footer.
I have a friend with a beautiful mint condition 2007 Baja 23 Outlaw single BB in perfect condition that is extremely easy to handle yet easily goes 70 mph. PM me if interested. He is ready to sell it to move up to a 33.