The Knight is a wonderful little mod, but I don't like to rely on stock coils so the RBA was a no brainer.
As others have said, 2.5mm seems to be around the sweet spot for coils on this RBA.
I've been running 2x30 core/38 wrap Ni fused clapton wire at 2.5mm diameter and that's about as big as I would go for more exotic wire.
You might get a 3mm coil in with simple round wire builds but once you hit a certain point you're choking the airflow.
Stick to 2.5mm :)
As always, coil choice/wattage/resistance is down to personal preference and how you like to vape.
As with any RBA, the key is in the wicking.
Don't stuff the juice wells with cotton, keep things nice and loose and trim the tails fairly short.