M_Boats: Split backstay adjuster purchase block sizes

Greetings, Since it is frozen here in northern BC and I have cabin fever, it has brought me to the point of making some rigging upgrades for my M17 - 032 s/v cerulea. I want to use a spiit backstay adjuster next season and would like to know what size purchase blocks fellow sailors are using to connect to the Johnson style bridle(west marine) without going overboard for the intended use. I figure this will help with luff tension on my forestay. I have a schaeffer snap furl - furler coupled with 135% genoa as my headsail. Thanks in advance, bruce.