I realize I should've asked a question, not added on to an existing question looking like an answer, whoops...

I am tapping into an existing 2" drain to add a washer, and the trap ended up pretty high in the wall to go above the existing sink connection on the other side of the wall. What's the absolute minimum drop you would use from the box to the trap? 18"? And/or secondly, how high up the wall is it reasonable to put the box (min 36", is there a max?)

speedball1

It's not the height of the washer box but the length of the standpipe. Too short and you'll have backups. The standpipe should be 2" and 36" in length from trap to washer box. if this puts the washer box a little higth the pump should handle the extra lift. Good luck, Tom