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You go by the manufacturer's recommendation.
It's called an axio swift backpack.
Recommended size is 14" to 15". You can go higher to 16" but not recommeded. Note that when you go for bigger rim, the profile must be adjusted accordingly so that the speedometer stay the same. Otherwise the speed recorded in the speedometer is incorrect.
According to SOWPODS (the combination of Scrabble dictionaries used around the world) there are 1 words with the pattern AXIO--G-. That is, eight letter words with 1st letter A and 2nd letter X and 3rd letter I and 4th letter O and 7th letter G. In alphabetical order, they are: axiology