I'm 6' 5" which, if I believe the advice, means I should be on a 29er. I rode one and hated it. Felt all wrong, especially when I got out of the saddle, and boosted my theory that the 29er craze is, at least in part, fuelled by reduced tooling costs on the part of manufacturers. But maybe that's just my cynicism.
I opted for a new 26 inch wheeled carbon hardtail at a time when everyone seems to be going the other way. Can't say I'm worried. In fact, I'm benefitting from being just as technically able as I was on a familiar set-up, and still overtaking plenty of 29er riding others whilst I'm at it. I got into and love MTBing for the technical challenge of the riding. Not stupidly big drop-offs, just tight singletrack, woodlands and the usual cross-country stuff. No doubt if I was good enough to be sponsored, or ride MTBs for a living I would be riding the 29er that my sponsors/employers want me to ride. The biggest determining factor when it comes to speed is legs, not wheels IMHO!