Anything above 1.6 mm should be fine. = 1/15 th of an inch

1.8 mm would be prefect = 1/14 of an inch

But 2.0 mm (2/25 of an inch) is most available for tubes of the outer diameter your looking for and will work well to. Don't go for 3 mm as that is only heavy and your than better of with a larger diameter 2 mm tube. Stiffer, lighter, and stronger.

Wouter