What size depends entirely on what you are using them for. I typically use something like 1000 100mm ties for replacing dead ties from previous years and for a few new props. I use some 150mm ones but barely anything larger. If I do need to go larger I will just join 2 150's together.
Using UV stabilised ties is important as non stabilised ones can break down within weeks of direct sunlight. Clear/white ties and probably most coloured ties won't be UV stabilised and the quality of the back UV stabilised ones varies which is why I replace a fair few dead ties each year.