Jonney,
Welcome to the PWN.
Three nozzle surface cleaners perform better
than the two bar models (clean faster) providing
you ave enough volume and pressure (GPM & PSI)
to power the surface cleaner you could use the
20" or 24" model.
Casters or floater is a personal preference. You are
correct in how to calculate tip / nozzle size.
My suggestion is for you to go with a 20"