what size of seatpost should i order for...
what size of seatpost should i order for 2008 trek fuel ex
Usually the size is stamped on the post somewhere near the minimum insertion line, if you have the old one around. Otherwise a dial caliper would be helpful in confirming the size.
But I'm pretty sure you're looking for a 31.6mm diameter post. Like 89% sure.
Length wise, you'll need to assess the way you frame fits or you could buy long and cut if needed. Good luck.