Most bikes when properly fit won't let you put your feet flat on the ground when in the saddle. One exception is the Trek Pure series, and that is a specific advertised feature.
I'm 6' even and I fit to a 20". That being said I ended up with a 22.5" 7100 (Craigslist deal) and am much the same, my leg extension is right when the saddle is all the way down. The only issue I have is the stand over is very close. To ball park where the saddle should be, sit in the saddle and put your heel on the peddle. Adjust so that your leg is straight when doing that. That way you will have a slight bend in your knee when pedaling normally. Riding lower than that can lead to knee discomfort.
It will seem weird at first, but long term it is way better for your knees. It helps when getting on to stand over the top tube, move a pedal to about 2 o'clock, and push on the pedal while hopping up. It will slide the bike under you, and has the advantage of taking off right as you mount the bike. I dismount similarly by putting one foot on a pedal at the lowest point, lifting up off the saddle, and stepping forward to the ground when the bike stops. After trying once or twice it becomes pretty easy.