what size skis should I buy

Depends on you level. If you are more advance, you want longer skis, slightly taller than you. If you are novice, you should get shorter skis. Talk to someone in a ski shop and they can hook you up.

Skis should be as long as you are tall.

Very rough guide,

They should come up to eye level. Unless you have an unusually large forehead in which case l do not know.

But as mentioned a ski shop should guide you right.

I'd shop around, you can get some great deals!