First of all, assume whose offset voltage to be 0, opamp or comparator?

As to the comparator spec., at least the followings have to be met:

1. Speed. Assume conventional sturcture, about 0.5*Ts for comparator to give correct results. Make sure comparator meets this requirement in Worst Case condition and leave some margin based on your power budget.

2. Offset. For conventional 1.5b/Stage, the maximum offset can be tolerated is +/- 0.25*Vref; Yet, we usually don't go to this extreme. 250mV offset is really big, even in CMOS.  100mV may be more paratical. Again, how much margin you can have depends on how much power can be taken by comparators.