I'm 6'1" and ride 23"/58cm road bikes. I have an old 19" '94 Gary Fisher hardtail that I wonder if I'm too big for, but I'm not really clear on mountain bike sizing variances. I will say I am having a harder time getting through technical terrain (especially climbing) then I remember from my last foray into mountain biking. Of course, both me and the trails are probably in worse shape (I bike 8-10 miles a day, but on the street with a road bike - totally different)
at 6'1" you should probably be on a 20-21" bike, somewhere around there, depending on the bicycle company.
but why are you having a harder time? if the bike is too small, it's not horribly small, and smaller is a bit easier to handle and maneuver. you probably need a slightly larger bike, but it also sounds like you just need to work on some skills.