Newman projections: how to tell which groups are bulkiest to determine what is the most stable conformation?

Well, I think your book is wrong. Read the extract:

A-Values are numerical values used in the determination of the most stable orientation of atoms in a molecule (conformational analysis), as well as a general representation of steric bulk. The utility of A-values can be generalized for use outside of cyclohexane conformations. A-values can help predict the steric effect of a substituent. In general, the larger a substituent's A-value, the larger the steric effect of that substituent. A methyl group has an A-value of 1.74 while tert-butyl group has an A-value of ~5. Because the A-value of tert-butyl is higher, tert-butyl has a larger steric effect than methyl. This difference in steric effects can be used to help predict reactivity in chemical reactions.

I guess what you're looking for are A-values. The higher the A-value, the more "bulky" the group is considered.

So, the $\ce{-SH}$,$\ce{-Cl}$ and $\ce{-Br}$ groups seem to be less hindering than the $\ce{-iPr}$ group.

References: A-value, Wikipedia