I have the FX3 in 25 and love it. My LBS ordered it for me with not commitment and when it arrived, they set it up for me including a raised neck on the bars. I knew I wanted it within 3 seconds of mounting it and the spin around the lot confirmed that the 25 was perfect. I had an old (classic?) Diamondback that was a 22.5 and while it was a great ride, a little tired, I thought I needed and deserved an new bike. I've only put a few hundred miles on it since taking ownership but it's been excellent.
I didn't get the disk model but I still feel like this bike size is perfect for me. I am 6'3" and mostly leg, like you, and this feels very good. You're welcome to come and ride mine if'n it'd be worth the trip to you, but I suspect you wouldn't...I'm a little surprised that you LBS would hold you liable for the purchase since they won't provide you with a test ride, but I guess that how some places do business. Good luck.