600x600 is a poor size because it is just over a power-of-two size (512x512) so will be awkward to assemble on to a power-of-two size spritesheet.
As the export-time optimisations tutorial says, the ideal size is two less than a power of two, because C2 adds a 1px border around images when spritesheeting. So on a 1024 wide sheet, 512x512 is padded to 514x514 and it can only fit one image on with nearly half the width wasted (unless something else smaller fits in there). However 510x510 pads to 512x512 and fits two across exactly with zero wasted space.
However those are some slightly weird numbers - it might be best to just aim for 2000, 1000, 500, 250, 100, 50.