play lots of maps with different circle sizes and form your own ideas for what cs to use in different situations
you'll likely notice that cs4 is more or less the norm nowadays so it's a good idea to start there; get a good grasp of how movement and playfield usage work on cs4 before branching out
some things to note:
lower cs value (bigger circles):
- aim is easier, so your patterns can be pretty brutal and still be reasonable
- everything takes up more space so playfield usage is less of a thing
- can apparently be harder to read in some cases but I can't confirm this
higher cs:
- aim is harder, so aim difficulty can get out of hand more easily if you aren't careful (especially important at cs6-7 or higher, since at those circle sizes stuff that's straining to aim isn't very fun for most people who don't play HR a lot)
- for this reason I'd suggest saving cs7+ for calm songs and using bigger circles for more intense stuff
- small circles don't take up much space, so changing the way you move around the playfield in different parts of the song becomes a more important way of creating contrast
- lower AR feels pretty good with smaller circles in my experience