For sBu and iPr this is easy. The Roman numerals I, II, III, IV refer to the "spheres" that are talked about in the Blue Book, which denote the distance from the stereogenic centre. Generally, this "distance" refers to the number of bonds: so Sphere I is one bond away from the stereogenic centre, and so on.
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
I've also seen them written simply as tBu, iPr, and nBu. They're abbreviations for tert -butyl, isopropyl, n-butyl (straight-chain), and cyclo -hexyl respectively.
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