Its always best to test ride, but if your dad doesn't ride much, I'm not sure he'd know if it felt right or not.
Road bike frame sizes are measured in centimeters. Typically that's the length from the center of the crank to the top of the seat tube, although some manufacturer's measure it a little differently.
MTB frames are measured the same way, but indicated in inches, or as S/M/L.
I'm 5'9" and my tri-bike (triathlon...) is 54cm, my MTB is 17.5" (considered medium)
If I could only have one bike, I would go with a mountain bike, just because it covers all the bases. I don't consider hybrids worthwhile because they're too much of a compromise. Not fast enough to keep up with group rides, but not good enough off road to handle singletrack. If I wanted a compromise, I'd go with a cyclocross bike, but I doubt your dad's ready for that.
As for brands, there are lots of good ones. Giant, Cannondale, Specialized, Trek to name a few.
Something like this wouldn't win any races, but would be more than adequate for general purpose riding:
http://www.performancebike.com/bikes/Product_10052_10551_1078331_-1_1512507_20000_400308
