Chemistry - Is there an abbreviation for "something bulky"?

Solution 1:

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

Solution 2:

I've seen numerous textbooks utilizing $\ce{R^*}$ ("R-star" notation) for a bulky group, and also even $\ce{R^{**}}$ ("R-star-star" notation) for "extremely"/"mega" bulky groups. From my understanding, star "*" symbol is supposed to resemble branched structure of the substituent.

Illustration on usage from [1, pp. 85--86]:

In place of a bulky substituent, we selected the chemically very inert tri-tert-butylsilyl group $\ce{Si^tBu3}$, called supersilyl and symbolized by $\ce{R^*}$. [...] In addition, the even more sterically crowded disupersilylsilyl group $\ce{R'} = \ce{SiHR^*2}$ and the extremely bulky methyldisupersilyl group $\ce{R^{**}} = \ce{SiMeR^*2}$, which we jokingly call “megasilyl”.

Note: An asterisk in the right superscript position next to the chemical element symbol also denotes electrical or nuclear excitation.

References

1. Silicon Chemistry: From the Atom to Extended Systems; Jutzi, P., Schubert, U., Eds.; Wiley-VCH: Weinheim; Cambridge, 2003. ISBN 978-3-527-30647-3.