I recently saw some ads for Allen folding bicycles. They claimed their use of bigger 451mm 20" wheels was superior to the standard 406mm. Let's go through their rational:

1. They claim the 451mm wheels are faster. 

Actually smaller wheels are more efficient up to 16mph, and then the efficiency is roughly the same. Bigger wheels have an advantage in efficiency over 33mph. Unfortunately there is no Allen bike that has gearing to pedal at over 30mph. Hence this claim is false, the 406mm are faster in real world conditions.

2. They claim the 451mm wheels provide a more comfortable ride: 

Actually the suspension in the wheels is provided by the tires. Unfortunately 451mm are not as standard, hence there is a very limited selection of tires. All on the market today are thin and provide minimal suspension. While 406mm wheels are the standard, and the market has tires in all sizes ( from 1.25" to 2.25" wide" ). Hence they are able  to provide more comfort if necessary. Hence this claim is false, 406mm bikes have more comfort choices.

That's not all! Buying 451mm tubes is hard, because no shops carry them. Additionally 451mm rims will be very difficult & expensive to replace.

In short stick with the standard 406mm on folding bikes. I wish Allen the best, but their sales pitch needs some changes.