6mm - this is the standard all-purpose rope size. Some people never use anything else. It should certainly be the basis of your rope collection.

8mm - this is the next most popular size. It can look better on larger people, and some people like to use it for the weight-bearing lines in suspensions etc. on safety and security grounds. It can give a different look & feel to any tie with any body. One disadvantage of the thicker rope is that is that knots can become bulky and ugly, so it puts a premium on good simple ropework.

4 and 5mm - these smaller ropes are good for intricate and decorative work on feet and hands, or head harnesses, or cock-and-ball work. They also make for an interesting different look with standard ties, but you need to be aware that the narrower the ropes become, the greater the chance of them cutting into flesh painfully and with possible bad effects. On the whole we’d say that you should leave anything less than 6mm until you’ve got a lot of experience behind you.

10mm - is bigger than most people supply, and really it’s too big for most on-body work, not least because of the bulk of knots etc.. But it does have some uses - e.g. thick rope can look good in spreadeagle ties and the like, and it may also have value in weight-bearing situations where, for instance, you might want to minimise the number of strands used. 10mm rope has over two and a half times the cross-sectional area of 6mm rope, so you should be able to use two strands of 10mm where you would otherwise have four strands of 6mm, and still come out ahead.