Any advice on how to go about sizing for 29er plus? Normally I'd just go with whatever's biggest since I'm 6'4" but just for the hell of it I'm thinking of adding a BD 29er+ SS to the stable and I'm presented with the option of a 20" and a 22". Looking at the geometry chart the 22" looks absolutely massive, with a 635 effective top tube on the 22". Any thoughts?

I just sold one of my road bikes today, and bought my first mountain bike since 2005. I am about an inch shorter 6'3" - and I rode a couple between 19 and 21. Ended up going with a 19.5 - just because the reach felt the best. I was right in between sizes, and I went with the smaller of the two options.

635 eTT sounds pretty normal for a size XL / 21-22" bike. I also would think that is the size an average-proportioned 6'4" person would want.

Just my $.02

Thanks, makes sense. I'm primarily a roadie so an effective top tube that long just sounds enormous. I have a long torso and relatively shorter legs so I ride a frame with a long top tube on the road but it's still only 605. But different fits, different bikes.

Yes. The biggest difference is stem length and bar reach. A 58-61cm road bike will have a 100-130mm stem plus when you're in the drops or hoods you're well out in front of that. A new MTB today will have 50-80mm and while bars keep getting wider, they're also swept back so your hands are nowhere near the position of a road bike.

I am 2" shorter than you are and I ride an XL Giant 29er that is 22". It fits perfectly. It does seem massive especially when parked next to my old 26er. They are just build differently. As stated previously you are not out over your bars so it is hard to compare them to road bikes. I do not like how wide the handle bars are though. It is not uncomfortable but it does get tight on some of the trails that I have been riding. I am contemplating switching them out.
I think that the 22" from BD will be fine.