Re: reel sizing for us warmwater guys:)
I've found reel capacity can vary greatly from one manufacturer to the next, and even quite a bit between reel models. One rated 3/4 may hold plenty of line & backing for a 4wt and another brand/model may be filled with just a 4wt line alone.
I tend to lean toward picking a size that will hold plenty of backing. For a 3wt rod I may go with a 3/4 or even a 4/5 if I'm unsure.