I know 18 sounds huge...but many longarms use a 20. :) 18 is the most common size used. In my longarm, I use a 16 the majority of the time, which accomodates most threads I use. But if I want King Tut or another thicker thread, it requires at least an 18, or a 20. The larger the needle, the less needle flex - on any machine. It's the flexing that causes breaks, skipped stitches, etc.
But the thinner the needle, the easier it penetrates the fabric.
