Make sure it's low-stretch
I agree that 3/8 is the right size for a 28.5' boat - but make sure the line is low-stretch (like the Sta-Set X that Jim used). My favorite is Samson's Warp Speed, which is made of Dyneema (a material that's superior to polyester for halyards, IMHO). It is exceptionally light AND does not absorb water (and therefore stays light). You'll want to order your halyards with an eye splice and halyard shackle. You can get this done by the Defender or West Marine rigging special services. For exceptional service, great advice, and super-fast delivery, I highly recommend Annapolis Performance Sailing (http://www.apsltd.com, 800 729-9767).