I am also 5’8″ and 150lb…I cannot for one minute imagine being comfortable riding a Shetland pony…I think my feet would drag on the ground! There’s nothing wrong with adults riding ponies,(Clinton Anderson only rides ponies which I find odd for a big man) but if you can’t sit comfortably, you won’t ride effectively. Many years ago I used to ride my daughter’s 13hh Welsh pony – everyone said I looked ridiculous but it was tremendous fun! If you want a pony, at your height and weight, I think you need a sturdily built one of at least 13.2hh to be able to ride properly at your height. Haflingers as someone else said are perfect for adults!