It really depends how much juice is running through the cable, for how long and at what length. You could go forth and do the math if the application demands it but honestly 18 awg (0.75 mm² for most people) should be easy to work with and will play nice with a good range of crimping tools.
If you want the cable to 'pad out' your sleeving material then you can opt for
UL1015. This spec is rated for higher temperatures (105 °C) and voltages (600 V) and so has a considerably thicker insulation jacket. This way, you can get good looking cables without the headache of trying to work with a guage of wire that's too thick for the application. Due to the increased thickness of insulation, the cable will 'remember' it's shape better
DATASHEET: 0.75 mm² UL1015 Datasheet
The 'standard' spec is
UL1007. This will give you moderate insulation thickness as it is rated for lower temperatures (80°) and voltages (300 V). Note that for a PC build, this is of virtually zero significance. The only determining factor here should be aesthetics and malleability.
DATASHEET: 0.75 mm² UL1007 Datasheet
Since you're looking for skinny cables however, you might want to check out
UL1061, it has the same temperature and voltage rating as UL1007 but has a much thinner insulation jacket.
DATASHEET: 0.75 mm² UL1061 Datasheet
Have a look at the datasheets to see exact diameters of insulation jackets.
Note that the cable stranded diameter of UL1015 and UL1007 are identical while UL1061 has a slightly increased guage for decreased resistance to maintain the same temperature and voltage rating with a thinner insulation jacket.
I'm yet to actually sleeve my Enhance 7660b but I've done the research, bought the sleeving and the tool (
IWISS SN-48B). I can't comment on how it crimps yet but it seems well made and I received a proper machined steel crimp die I ordered seperately also from aliexpress and not the bobbly black metal ones that I assume come out of moulds.
And that's my long winded story about cables that nobody asked for or probably wants, haha, hope you liked it.