Are all the rubber seals on thresholds the same size? If not how do I figure out the right size?
Nope. Many many different sizes.
I'd take the old one to the home improvement store and match it up.
I've spent days looking for the correct seal a few times.(rental properties) Always optimistic I would get lucky this time. Never. The other option is a wipe seal you attach to the inside of the door. With wood floors it is easy to get them the right height to seal against the threshold but not drag on the carpet.
The dent in your threshold may cause a gap though. Looking close you might want to check the gap at the bottom of the door. The scrapes on the aluminum mean the door is dragging and shearing the rubber seal.
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