I found some people are using 1/4" diameter butyl tape and some are using 5/16" diameter butyl. What the correct or best diameter to use on the 914?

I used 3/8" on bare naked metal. 1/4" definitely too thin, unless there is already a bed of old butyl.

here's my install
http://www.914world.com/bbs2/index.php?showtopic=235156

Techniacally 10m but 3/8 will do -

Someone said they used 3/8" diameter butyl and could not snap on the windshield trim. Is that possible?

If there is a pre existing bed of butyl, i could see that as probable. My installation was pretty much the correct height with a bare frame.