Is there an abbreviation for "something bulky"? I want to draw a scheme that indicates the difference between the products formed in a reaction with a molecule that will form one thing when it has methyl or hydrogen substitution, but another thing when it has any substituent more bulky than isopropyl. 

Is there any abbreviation (like "R" or "X") for "bulky"?
I've never come across one, but mostly those I come across will have "R" or "X", but somewhere in the reaction scheme specify what R is, like:

R = Me, Et, Pr, iPr...

So in your case, you could specify something like:

R = iPr or bulkier