Most yoga mats are 72 inches in length. That means a person who is six feet tall should be able to lie down on it just fine, although it would be pushing the limits. If you are a taller than this, then you need to purchase a long yoga mat instead.
You can get these in 84-inch lengths or even longer. That should be more than enough for the vast majority of people. Check the width as well since the regular 24 inches might be too restrictive for you. Opt for 36 inches or even 48 inches if you need to.
Taller and wider people will generally weigh more as well thanks to their bone structure. You might need a thicker yoga mat to get enough padding against the ground. Don’t go any lower than half an inch for comfort.