Originally Posted by
chaadster
If you’re between sizes, always go smaller. No question…but I guess that’s not the question.
To the question, generally speaking, 5’9” sure sounds like a Medium size frame.
I disagree. It depends on your riding goals and the specific bike in question. At 6' tall I'm often in between a large and medium frame and usually go with large. Why? Because I'm primarily an endurance rider and prefer the inevitably longer wheelbase and higher stack of a larger frame size. If I was racing crits I may well choose to size down for a more aggressive position with slammed, super-long stem. But that's not how I want to ride all day.
But I totally agree at 5'9" I can't think of many Large frames that would work well. I would have thought the OP was a medium without much question.