What's the best size hoook?

It would really depend on how big you want your project would be. I'm from the Philippines, our hook comes in numbers not letters like in the U.S. I usually use hook number 8. I am not also familiar with the crochet cotton that you are using so is really hard to give suggestions.

I use a size "H" when using U.S. 4ply cotton. Cotton I have found can be tricky, depending on what your making. It will shrink some when washed, and it stretches. I made a pair of slippers using cotton and they fit great. But as I wore them they kept stretching. Had to re-make them using acrylic.

I am from USA, and worsted weight would be D or larger. I prefer a J to do much of my work. It is loose but not real loose. I do not have to fight with my work, but it is not lacy looking.