The length of the bombilla to choose depends on the size and style of the gourd. The straw needs to be long enough to go from the bottom corner of the gourd out the top of the opposite side so that one can manage to sip at least a few inches above the gourd's rim.
There is no bombilla length limit, yet if the bombilla is too long then it could tip the gourd. For this reason some will seek out a base on a gourd that will increase the stability of the yerba mate gourd. Consider that heavier bombillas are more likely to tip a gourd that does not have a stable base.
The straight bombilla is the traditional preference because the drinker is able to stare into the yerba mate, which is a way of respecting the value of the herb. Further, the straw is placed angled in the gourd, which makes a 'straight bombilla' come out at a slight angle. The curved bombilla allows one to look forward as they drink yerba mate, and was originally created for people to use while driving their car.