Will this replace a regular wood folding attic stair system where the ladder was built to the door? How do you know which size to buy?

The height is measured from the floor to the ceiling. This ladder mounts to a header on one side of the opening. The door is free of the ladder. It is mounted with its own hardware. When the door is opened, the ladder remains in its collapsed position. Then the ladder will be pulled down by the assist pole after the door is safely opened.

This ladder is fixed to the ceiling joist, not the door. You will almost certainly need a new door. The ladder can be adjusted to fit your ceiling height exactly, so long as your ceiling is no higher than a maximum of 10' 3": there are very clear instructions supplied with the ladder.