I have purchased and own 11” foam planes, 13.5” planes, these 17.5” planes, and the large 20” planes. Out of the four different sizes, these seem to fly the straightest, are the most predictable and glide very nicely.
The smallest planes are extremely fun, especially in the wind. They have potential to do crazy things due to their super light weight. The 13.5” planes are okay, but mine aren’t very fun. The 20” planes are also fun and fly quite straight, but they don’t glide as well due to their somewhat heavier weight and not too much more lift.
These are my favorite size, followed by the smallest and largest, and finally the 13.5”, which don’t do much for me.