What a question!
Well, it all depends …..
Firstly and above all your rope must have a breaking strain fit for its purpose. If it is to be load bearing, it must have a breaking hugely in excess of any anticipated load.
Secondly, the rope must feel comfortable to use.
If you use poly rope it should not feel rough – the rougher it is the more likely to incur rope burns if it slides over skin. All poly ropes are more susceptible to causing friction burns.
For general purpose “just tying up” I would probably suggest you start with a few lengths of natural hemp 6mm rope. Natural hemp has a nice feel, is generally relatively smooth and less susceptible to causing any rope burns. Depending on the ties you will want a selection of lengths. 10m for complex ties and shorter for more general use.
Make sure you have something handy to cut the ropes in an emergency and make sure you know a good range of suitable knots remembering that where there is a loop around a body part it should not become more constricted under tension.
Listen to the sub – they will know when it becomes “too much”; “painful” and potentially dangerous.
Keep googling as well – there is a lot of good advice out there (and some poor advice… be warned)