On the pack it should say. As i recall the sizing is for height rather than size, apart from xl which is medium for larger ladies. I grab a pair of black knickers or boy pants and put them on over the tights, holds them up a treat :P
On me, that means they are too small (or at least, too short in the leg).
My foolproof solution is to snip through the toes, and wear with popsocks or kneehighs to give an expansion joint at the ankle - works with everything except fishnets or crochet tights, for obvious reasons. And posh tights now last forever because they never wear through at the toe or heel .
And only works with boots, unless you don't mind looking like you're wearing ankle socks over tights.