Originally Posted by markhr
The best you could do is give him a cash/gift voucher to a good bike shop - it's better to ride a bike that's too small than too large (especially where knees are concerned). Bikes and bike saddles are like shoes - you don't know it's going to fit until you've tried it.
I agree that the gift certificate idea is a good one. I also agree that bikes and especially saddles are a matter of personal preference.
I strongly disagree, however, with the statement that too small a bike is better than too big a bike. First off, your dear ol' Dad needs a bike that is at least close to the right size for exactly the reason you mentioned, as the most common ways of screwing up one's knees on a bike are (1) seat too low, a natural result of a too-small frame and (2) pushing too big a gear too much of the time - training yourself to maintain a cadence of 70 rpm+ (80+ is better, but not everyone likes it) and shifting gears to keep that cadence as much as possible.
But assuming your Dad settles on a particular model and finds that his ideal size is between two of the stock sizes (should be no more than a 2cm to 3cm difference between the two sizes), I would advise going with the larger size. Why? Because it is a heck of a lot easier to make the larger frame size fit right via adjustments to the seat and swapping out the stem then it is to make the smaller size fit right the same way. This is especially true of getting the level of the handlebars where you want them - with the smaller frame, it is much more likely that Dad will be stuck with handlebars that are lower than where he would be most comfortable.
The cyclocross bike idea is a good avenue to pursue, but I would not make it the only one. CX'ers are built to take off-road abuse, but they also tend to have a higher bottom bracket for off-road ground clearance, which puts the rider higher off the ground (duh), which means a slightly less stable ride compared to a similarly sized bike with a lower bottom bracket. Some riders prefer the higher BB, and some can't tell the difference, but some think that a high-bracket CX machine feels squirrely compared to other bikes (usually road bikes) with lower BB's.
Another good avenue to explore is touring bikes, such as the venerable Trek 520, intended to carry a full load of panniers, sleeping bag, tent, and other camping gear. Such machines are already built with carrying a heavier-than-usual load in mind, and may be just the ticket for Dad.
Finally, as a 285 lb Clydesdale myself, don't automatically assume that Dad can't enjoy some of the standard road or mountain bikes out there. Check out the Clydesdale section of Bike Forums for discussions of what we big boys can make use of.
Oh, and kudoes to you for encouraging your Dad's bike lust. It is a tremendous sport, one that can be enjoyed damn near forever at pretty much whatever level of craziness (or lack thereof) that fits anyone's particular bill. You can do it alone, you can do it with friends, it can open doors to making new friends - it's simply a lot of fun. I hear it's good exercise, too . . . .