The standard ones will be 7/8". Any Renthal or similar bars with a cross brace/bar pad will be the same.
'Fat Bars' or taper bars will be 1" in the middle (at the clamps) and 7/8" at the ends. You'll need to replace the clamps or fit adapters if you go for these (although the adapters may come with some brands of bar)
For handguards I use Barkbuster Storms, because they have massive weather/flying-bits-of-rock protection, and a fairly hardcore ally spine. That's for a Tenere that gets used all year round and off-road, so you might want something a bit more discreet/stylish on an XTX
Again you need to decide what bars you want, then make sure you buy your handguards with the appropriate fitting kit for 7/8" or 1" bars.