How tall is this item?

I do believe it’s the standard width of a patio door

Can anyone confirm that this is reversible for doors that open the opposite direction?

Hello. In response to your question, the draft stopper is fashioned in a " tube" out of a soft, sturdy flexible material and filled with some type of "beads". It rests in the sliding door track and can be "folded over" if it is too long. It is nicely made and works well to keep cold air out. As far as the doors opening in different directions, it would appear that the draft stopper would have to be removed from the sliding door track in order for your sliding door to open. I hope this information will be helpful.

Is it simple to shorten them if necessary?

I did not try this since it fit my door, but since it is made of a sort of fabric, I would imagine you could carefully cut open each end (being very careful that the tiny beadlike filling doesn't spill out), then cut, tape shut and sew. You would have to measure each half of your door and probably do both ends, since half of the item is double-wide to fit the indented half of the door. You'd want to be sure not to shorten too much, because each end needs to fit snugly against the door frame to keep out drafts.