I am (nearly) 6’5” and running the 61 cm with a 130mm stem. But this doesn’t tell you anything very useful.
The best way to know is to compare it to the reach of your current bike:
- Do you have the fit dialed on your current bike?
- what stack and reach does that bike have?
- How does that compare to the 59 and 61 Warbirds’ stack and reach?
If both sizes seem reasonable (for example a 85mm stem on the 61 and a 100mm stem on the 59), then I would go for the smaller one, since it will have a better weight distribution, with more weight on the front wheel for traction.
I have a Salsa Colossal that is almost identical in stack and reach to the 59. That bike works well for me, though I end up with 35mm of spacers and a +10 degree stem. Really, I'd like the stack a little higher. The 61cm gives me +19mm on the stack (perfect), but +8mm on the reach. The guy at the bike store said I'd need an 80mm stem to make that work. I'd also be a little worried about the standover on the 61cm.
I think I could make both sizes work, but I'm leaning towards the 59cm.