Most rules of thumb will say that for your size boat, 3/8 rope and 1/4 galvanized chain rode is adequate. I am a big advocate of OVERSIZING rodes and anchors. Don’t go nuts, but I would choose a rode and anchor that is rated for 150% of your displacement and length. The web is full of free rode/anchor guides. Im sure West Advisor at Westmarine.com has one.
If your are building your own, err on the size of bigger (as in 1/2 and 3/8 as you mentioned). Also get an adequate anchor. There are a lot of “recreational” anchors (generally the type that are designed to be easy to retrieve) that will place a disclaimer “not to be used in critical situtations.” That phrase tells you everything you need to know about them… avoid them. Unless you fish only small calm farm ponds, you need an anchor that can be depended on to stop and hold your boat in ANY condition. This equipment is not just convenience, it is safety. Some of the rinky-dink specials that people use on large commercial traffic waterways are insane. You have to ask yourself, “Are you confident that your anchor will stop you dead and hold in all conditions? On a fast moving river? On a windy shore?” If it can’t, you need a better anchor and rode.
This is a serious piece of safety equipment and is your LAST line of defense against ending up against riprap, wingdams and other boats should your engine fail.
Sorry for getting preachy… but I’m USCG AUX and I spend a lot of time reading boating horror stories… ah, the things that could be avoided…
Good luck,
Dave