Set ATC speakers full range no matter the size
Not for AV I wouldn’t, personally. Regardless of the speaker, set to small and choose a suitable crossover point to the sub. Eases the load on the speakers, helps them sound better, allows them to go louder - let the sub (and amplifier) deal with the demanding stuff. It’s just a case of finding whereabouts the bass starts tailing off in the room they’re being used in. A more capable speaker allows you a lower crossover point, but you have to weight up whether each 10Hz you drop (for example) is better produced by the speaker or by the sub. The sub will normally be more efficient at reproducing it, but it will be localised to a single point, whereas the speaker output will be weaker, but localised to the speaker producing it. Ideally you want as low a crossover point as the speaker will allow.
The speakers I’m using at the moment would be rough equivalents to the SCM7s, and whilst I have them set to large - and as impressive as they are - I would be scared to turn them up too loud. It suits my listening level though.