does anybody know if i can put a 16" wheel on my 2003 heavy duty

 

I don't think you can, the brakes are too large.
Go 18"

 

Will not work. Some aftermarket 17" rims won't fit either, it's a brake caliper clearance issue.

 

Ditto that! I've got a beautiful set of 17's sitting here, waiting for the fed-ex truck to pick them up and return them to the sender. They won't clear the calipers by at least 1/4 inch. The safest thing to do is get the wheels from a local shop so that you can confirm the fit.