I have a 15" SDM and the eyepiece height is
juuuuuuust right for me standing up when viewing at zenith.
That wasn't an accident or even good luck. The scope was designed that way just for me
SDM were happy to do it for me, so they would for you.
The typical 18" sold by obsession et al is usually a f4.5, requiring a small step to the eyepiece at zenith. No big deal really. The next one up is a 20" f5, requiring a ladder. It
doesn't have to be f5 though!
Peter at SDM is very flexible. You can choose what you want within reason and Peter will build it for you.