I think chapters vary. Um.. Kenshin, for instance, is usually 9 chapters of 20 pages. Fullmetal Alchemist is 4 chapters of anywhere from 35 to 60 pages. In general, I think the books are about 200 pages long.
Intersting. 20 pages is about the same as a single issue of a US comic. 35-60 would be similar to a "giant-size" format that was popular in US comics for a brief period but fell out of favor because it wasn't cost-effective; and curiously, in this format the comic was usually divided up into chapters and/or multiple stories.
I'm checking out the FMA site now. It's actually downloading at a reasonable speed :)