A 20" bar is very effective. However a 24/25" bar is nice if you want a bit of reach for limbing and as long as it oils well enough you will be fine just might be a touch slower in some situations and bog a little easier. You can always latter add a 20" for a reasonable price. It doesn't need to be a stihl bar just stihl mount.
@fordf150 has total bars at a good price and you could even go cheaper with a Forrester or Bailey's woodland pro. 60cc is pretty versatile saws and having two or more bar lengths come in handy. Especially if you pinch your bar you can take remove the power head put on your second bar and free it. Thankfully that is easily done with the inboard clutch. With some saws you have to remove the stupid outboard clutch to do that. (Poke poke)