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No problems with 6.5" speakers.

Coaxials (multiple speakers in the one bracket) are an easier but worse option when compared to separate components. If you can be bothered to install separate speakers for the woofer and tweeter I'd recommend going with that; if you can't be bothered, 2way coaxials are often less bad than 3ways.

I've found that 6" is probably close to the limit.

I've got 5.75" Scanspeak Revelators in there right now on custom baffles, and there isn't a lot of room left diameter-wise. That said, my baffles are cut for a flush mount, so that takes a bit more space also.

Without a flush mount cutout i'd say you would probably squeeze in a 6", maybe a 6.5" - anything bigger then that and you'd need some custom pods (unless you dodgy install it, which i wouldn't recommend).