Okay so base size is the number that decides whether a topper actually covers what you need covering, and I'd sort it before looking at anything else on the listing.
It's the measurement of the part that sits on your head, in inches. My smallest topper is 5.5 by 5.5 and my biggest is 9 by 9. Written down that gap looks like nothing. On a head it's enormous.
My Claire sits on the crown, does the parting and stops there. The Melanie and the Jon Renau cover most of the top of my head, and they start to feel closer to a wig than a topper.
What caught me out is that two brands quoting one number aren't always quoting the same thing. My Moonlight has an 8 by 8 cap, and the hand-tied silk part inside that cap is 4 by 4. Those two numbers do different jobs. The 8 by 8 is the footprint that has to cover your thinning. The 4 by 4 is the part that looks like scalp, and the only area you can move your parting around in.
So two 8 by 8 toppers from two different brands can be doing completely different jobs. Check what each number on the listing refers to before you buy.
And measure your own thinning first, because the base only helps if it's bigger than the area you're covering. It only takes a couple of minutes (and it's the step I'd never skip now).
All eight of mine, from 5.5 by 5.5 to 9 by 9, are side by side in my best hair toppers UK post, and the measuring is in how to measure your hair loss area for a hair topper.