If you're not sure and/or haven't heard these before, stick with the 6.5" so you have an easy upgrade path.
The cutout sizes for in-ceiling speakers vary quite a bit from vendor to vendor. And 8" speaker might have anything from 8.5-11" diameter cutout. The issue is if you decide to upgrade the speaker - you have to pick one with the same or larger cutout size, otherwise you're doing a patch job.
Because any 8" speaker is likely to have a larger cutout than any 6.5" speaker - upgrading from a 6.5" to an 8" (of any brand) is easy. But starting with a cheap 8" one (and cheap ones tend to have large cutout requirements) will limit your choice of upgrades, even among 8" models.
I started with 8" throughout the house because the price delta wasn't large. But the performance of those speakers is "meh". When I upgraded (by moving speakers to other zones I was adding), I only had a few choices for higher-quality 8" models. Worked out ok for me, but hence the advice...