when they first came out, almost all of them had 20"s. I got mine as the 2003 were coming out and the dealer I purchased from only had 4x4 Regular Cabs which had 17" wheels. I never wanted 4x4, but I absolutly didn't want an extended cab, so I got the smaller wheels.
I thought about upgrading to stock 20"s but those things cost right under $1000 dollars per tire & wheel combo. And I really don't feel like spending that kind of money on a vehicle I'll probably only have for another year.
Especially with the new '05 Mustangs coming out, I'd really like to pick up a Cobra when this body style ends and prices are cheaper.