I tend to ride bikes a bit bigger than I should, but in my opinion at your height your a large in Specialized bikes, and a 16" anything is def too small.
5' 8 and ride an 18" Scandal (according to the bumf I should be on a 16") and I had a Med Stumjumper FSR, I spent a bit of time fettling and had the cockpit and saddle to crank (height and for and aft) set as close to identicle for each bike as I could get (really close as it happens within a couple of mm)
I took a large Spesh Enduro to the Alps and that was fine, though def a little bit big, better in my opinion than going a bit too small.
Rob what size SX trail and what model? Wouldnt mind one of the very early white ones from about 04-05 I think if one came up at the right price.