I am 5ft 10 and sz 12. Also longlegged, not surprisingly. I have a nightmare finding tights that fit.
Size "large" tend to be too long and/or too big and baggy. Anything less just leaves me with a crotch between my knees and toes that are crushed.
I have wasted sooooo much money trying to find the perfect pair. Today, in BHS (normally ignored by this fashion victim ), I did.
The size large, 100 denier cotton ones fit me perfectly. The waist is on my waist, the crotch is where it should be and they don't fall down around the knees. Thy also have brill colours of black, chocolate, grey, navy and a faaaaab plum colour. All for a fiver !!