@makai: Zépir's War3 Map Editor (
link) might be able to. I haven't checked. I haven't tested it.
Anyway, about your original question: each pathing blocker takes up 50 bytes (uncompressed). Using simple conversions:
1 kilobyte = 1024 bytes
That means that it takes 21 pathing blockers to take up 1 kilobyte (1050 bytes). More conversions:
1 megabyte = 1024 kilobytes = 1048576 bytes
That means that it takes 20972 pathing blockers to take up 1 megabyte.
How many pathing blockers are in the average map? It really varies. Let's take, for example, the Diablo III Warcraft III map. It uses 2324 pathing blockers. That is 116200 bytes, which is 113.477 kilobytes.
BUT! Warcraft III uses zlib compression, which results in the doo file being usually ~1/3 the original size. So in the end, it takes up probably ~38 kb for 2324 pathing blockers. And on that note, you can approximate each pathing blocker as taking about 16 to 17 bytes each. In the end, you shouldn't have anything to worry about.
In case you're curious, using the pathing map directly would yield 4 bytes per little pathing blocker, and 16 bytes for a larger pathing blocker. It compresses extremely well too, since the wpm tends to be very repetitive. But again, for practical purposes, it'll probably save you only 5-40 kb.
Off-topic: It would be a neat idea for a tool to read the war3map.doo, find all the pathing blocker locations, and then write them directly to the wpm (and then you can delete the pathing blockers afterward). It might not help the file size much, but perhaps it'll improve save/load times (for editing, mostly). ;D Although, I think the editor recomputes the pathing map each time you save, so there may be some complications.