^^. What is the digital looking unit by the hose reel?
To the OP, 5/16" for what we do.
1/4-20 for inserting into wood... not really rivnuts... the kind that screw in.
8-32 and 1/4-20 for rivnuts. I probably use ten times more 1/4-20. I have #6 and #10 but rarely use them, if ever.

How do you use a Rivnut http://www.rivet-nut.com/Rivnut_Fasteners.asp
in wood? Im thinking you're referencing a different product
EDIT....Ahhh, sorry i see you qualified the statement, weird, my brain did not read that the first time..
Like these from EZ loc id guess
(snipped images)
