Do you have to sit cross-legged on the floor to meditate?

Not at all. You can meditate in a chair, lying down, standing, or walking. What matters is a position you can hold comfortably while staying reasonably alert, not a particular shape. The cross-legged image comes from tradition and from photography, not from any requirement. Pick whatever your body is happy to hold for a little while, and change it whenever that stops being true.