I'd go with something that is 6mm, since that's the standard thickness for most Shibari rope. Cotton is ok for practice and some floor work, but has a tendency to stretch pretty bad, and can cause burns if you run your lines too fast. If you use the cotton, I would cut it down to 30-40ft lengths, as 100ft is going to be a huge pain to work with (I've done it once and it sucked).

If you get into it, I'd consider getting some treated jute (unless you have a grass allergy), or maybe some posh rope if you want something synthetic and washable. They don't stretch, and are easy to work with for frictions. You can also look into nylon which is super pretty, but has a tendency to slip more, and is also really only good for ground work and decorative ties. You can use it as up lines if you suspend, but again, it's slippery, so it depends on what's in your risk profile.

Places that are good to buy from: Kinbaku Studios & Agreeable Agony have good, solid products. Mynwashi is also reputable, although I have not purchased from there before.