5/16 seems to be chain of choice for your boat, Marshall. At least that's what I've seen over and over again as cited as the chain folks are using. It'll be a bit lighter than 3/8. Note, however, that the gypsy on the windlass needs to fit the chain properly. I'd suggest that you check to see what size the gypsy is - and then either use that size chain, or get your chain of choice and change the gypsy if it's not the correct size.
I've got an IP32 also, and have 200 ft of 5/16 ISO grade 43.