While I know it is the larges route between Skyrim and Cyrodiil. I don't think there is any source saying how long is it. So while I believe it is roughly a few kilometers wide and long, I want some opinions about it.

Do we know where it starts or ends?

I don't think so. This question I ask would mostly be answered by mere speculation or guesses. I just want the opinion of lore experts on the size.

Well, best bet would be to try and figure out where it starts and ends in Skyrim and in Cyrodil (and maybe ESO if they have it). Then take either the distance from Almalexia to Vvardenfell (250 miles) or the average distance from Lilmoth to the sea (about 15 miles). Then just find about how much the length of where Pale Pass should begin and end, and compare to the previous measurements and estimations.

That is the best answer I can give, and I doubt anyone off the top of their head knows the length of Pale Pass in here.

Hope that helped! Good luck!

The distance between Almalexia (Mournhold? Or is their a city named after a her? I don't much about Morrowind's cities outside Vivec and Balmora.) seems to be waayyy longer than the length of Pale Pass. So I would guess 5-10 miles would be it's limit. Perhaps I should go find a Map of Tamriel and take a look at Pale Pass.

Thanks for the help though.

Yeah, Mournhold is Almalexia if I remember correctly. Also, Lilmoth is an Argonian city. They aren't proportionally the same either (I checked), so allow for a certain percentage of error. I wish I could be more helpful, but I'm not very good at the cartography stuff.