I usually recommend a boat-length of 1/2" chain, and an anchor between 12 and 25 lbs. It makes a huge difference in choosing a brand/model of anchor. Even within a company, say Danforth, there's a big difference between a basic Danforth, and a Danforth Hi-Tensile.
A basic Danforth: a 14# anchor is rated for a 31 foot boat.
Danforth Hi-Tensile: 5# anchor is rated for a 31 foot boat.
Nonetheless, I usually go with overkill. Our 2520 has a 25# Delta, 30' of 1/2" chain, and 600' of 1/2" nylon 8-plait rode.
Our 42' WesMac has a Navy 105#, 50' of 3/4" chain, and 900' of 3/4" nylon 3-strand.
Plus, bottom type will determine the best anchor for you. If its rocky, a danforth-style won't hold unless it gets wedged. If its really muddy, it'll drag and you'd be better off with a plow-style anchor.