Is the size of the rope important in Shibari? Yes, because there ropes of different seizes and each one of them serves a different purpose. For Kinbaku, tops use equal length ropes of 7m-8m. This is because that size of rope is actually easier to handle.
The size and length of the rope are personal. The only recommendation is to ensure the rope is no more than 4 times the span of your arms. That way you can pull the rope in one movement. If the rope is too long, you will need to pull too much rope in every knot you make. If you think you might need more length in certain knots, you can always tie another rope in order to make a larger one, and you will be able to unknot the rope it will save much pulling through of rope and will speed up your work.
Regarding the thickness of the rope, it is usually around 5-6mm, which is used for suspension, and a thinner cord around 4mm is often used for floor work or detailed bondage. I think anything thicker than 6mm becomes very hard to work with and it will result in bulky un aesthetically knots, which are ugly and will deffinately is not comfortable for the bottom. A thinner rope will also be uncomfortable and will deffinately cause more damage than a thicker one. Although the size of your rope is your choice, the recommendations of the rope are between 4mm to 6 mm and between 6m to 8m. The standard Shiabri ropes have emerged from a lot of experiences, so there is a good reason why they are the way they are.
So, the answer to the question is the size of the rope important in Shibari is yes.
