There are lots of good suppliers for bulk line, including our hosts here at SBO. IIRC, I got my 1/2" nylon 3-strand from Jamestown. Or you can wait for WM to put their line on 30% or 40% off, which they do occasionally.
I went for 1/2" over 3/8" partly for a little more strength, but more for abrasion resistance. If I had it to do over again, I think I might look for a 7/16" 12-strand (like New England's Mega Braid, although I don't think they make that in 7/16"). Still larger and stronger than 3/8" 3-strand, but easier to fit in the anchor locker without twisting and tangling. That's my guess, anyway. I'm not certain if any reputable manufacturers make it in 7/16", and I'd better not actually go look for it, or I'd be tempted to spend more $$.
Re: Chain: As
@CloudDiver said, the standard rules seemed to be either 2/3 or 1 boat length of chain. So, being my paranoid self, I compromised on ~1.5 boat lengths (30') of 1/4" high-test. My memory is that the 1/2" 3-strand spliced fine to 1/4" chain. At least it looked OK as I watched my wife work - she's much better than me at splicing 3-strand.
