The size you need is going to be dependant on the type of torch you have. There are size differences between manufacturers.
Take a look at this chart:
http://bikesmithdesign.com/Welding/Tips.pdf and see if your torch is listed.
As an example, I have a Harris torch and I use a #1 tip for 18g (~1.2mm) and #3 tip for 16g (~1.6mm). This is for sheetmetal, with aluminum the tip sizes can be larger.
Hope this helps.