I'm 5'10'' and ride a 15.2hh and a 16hh (16.1hhish, he'll get properly measured again soon, last time he wasn't mature and measured 16hh). I use a mounting block to get - I actually got a pair of painting ladders from B&Q, it's a long metal bench type step thing, which is fantastic for moving to the horse rather than trying to make the horse stand next to it. Also light weight and easy transportable.
I'm so unfit at the moment, I've been very sick over the past few years, and have put on weight and lost so much fitness, I want to be able to mount from the ground, at the moment it takes a million attempts - my mare gets annoyed at it, and my gelding just tends to give me a dirty look then plods off.
I also used a block to mount my 13.3hh pony when I had her, many years ago, I was also shorter, and rode bareback most of the time.