seat stays 5/8"
chain stays, 7/8" -- will probably require bending and definitely require crimping so standard bike tubes might be better
Top tube, 1 1/8"
Seat tube, 1 1/8" (get some 1 1/4" .058 wall for a sleeve at the seat cluster)
Down tube, 1 1/4"

The top tube and down tube can go up or down 1/8" depending on what you want the bike to look like. Bigger will be stiffer/heavier
Probably best to go with 1 1/8" seat tube for seat post compatibility, although you can go up 1/8" and sleeve for the seatpost