Cis-1-tert-butyl-4-methylcyclohexane exists almost exclusively in the conformer where the bulky tert-butyl group is in the equatorial position, and the smaller methyl group occupies the axial position. This conformation minimizes steric hindrance between the bulky tert-butyl group and the hydrogen atoms of the cyclohexane chair structure.
The fact that this molecule prefers one specific conformation tells us a lot about the relative sizes of the tert-butyl and methyl substituents. The tert-butyl group, which is significantly larger due to its three methyl groups branching off the carbon, requires more space and creates more steric strain when placed in an axial position. In contrast, the methyl group, being considerably smaller, can comfortably fit in the axial position with less steric hindrance.
In summary, the preference for the conformation with the tert-butyl group equatorial suggests that it is much larger than the methyl group, as larger substituents will favor positions that reduce steric clashes in cyclohexane rings.