Some other considerations: how tall are you? You won't get headroom on a small boat. Or a head. Or a galley. You have to decide what you can live with.
Or rather, what can you live without. A 21 footer won't offer much as far as amenities go. Again, it won't be much different than camping.
My 30' Catalina was a nice boat, but my 34' Catalina offers a lot more. A gas stove/oven as opposed to an alcohol stove. A refrigerated ice box. A water heater. More battery power. More fresh water storage.
And actually, my 34' is much better equipped to single hand than my 30' was. Of course it is a newer model and came from the factory with more options than the 30 did.