Well, it depends on the thickness as well (thicker the rope the more you need).
What I would personally recommend is the following
Wrists:
Legs:
Body:
- 8mm diameter
- 10m length
- The length is a suggestion and is affected by many different factors, but 10m should be easily enough for a upper body harness.
Now, then as for the change in diameters, 8mm will be very confterable and will spread the pressure nicely, while in comparasion the 6mm will be harder to escape from but should be tied slightly looser.
As for how tight to do them, people normally follow the rule of 1 fingure gap, so if you can get a fingure between the rope and the skin or cloths without digging into the person, then it is as tight as it should be. As you get more experience you will naturally tie it as tight as you/the subject like it without needing to test it.