We recommend starting with the "Starter Pack", which gives you one of each size - Large, Medium & Small. Use the PerformAIR Starter Pack with all three sizes included to determine the best fit for your nose. PerformAIR is highly adjustable and you can custom configure it to fit your nose.
Once you've found your most comfortable size and your breathing is greatly improved, you should consider picking up a "Refill Pack" in your size, which also comes with three re-usable PerformAIR devices.
Each PerformAIR can be used for about 10 days, so one pack of three (3) will last about a month.
Both Packs also come with a handy little storage / travel case.