Whatever is appropriate for the map.
Sometimes I see big circle maps where the circles overlap in an ugly way and I swear that the map was intentionally written to be played HR, because it looks so much better that way. Big circles can also lead to patterns banging against the walls, making things feel cramped... for some songs that works, for others it interferes with the flow.
Small circles work well if you want long flowing patterns and to keep things clean. However, I've also seen maps where small circles were used and only about 70% of the height and width was used... which is 49% of the play area. Sometimes it's not so obvious that the space is being badly used... the map might have notes on the wings, but the area used in any section is a small part of the screen, it never really needs the full space (this is often the case with size 6 maps, which often seem to be Hard maps where the very small circles are used to up the difficulty (the Insane will often have larger circles but a lot faster movement)). This is typically a sign that the map might have been better with larger circles and beat spacing.