Zipp recommends between 23-25 width tires on their website. The rim is built for that width.
from the zipp FAQ:
Tire choice depends highly on user preference and conditions. To summarize, a 21mm has superior aerodynamics with our rims; a 23mm is larger and subsequently has better ride quality and rim protection, but at a slight aero penalty. Here are some questions you can ask that will help guide your decision:
Triathlon/Time trial? In general – 21mm.
Road racing? In general – 23mm.
Training and/or daily riding? 23mm+
Dry? 21mm at normal recommended pressure.
Wet? 23mm at a slightly lower pressure.
Smooth roads? 21mm at normal recommended pressure.
Rough roads? 23mm at a slightly lower pressure.
User prefers slight aero benefit of 21's at the expense of a little ride comfort, rim protection, and rolling resistance? Use 21mm.
User prefers slightly better ride comfort, rim protection, and rolling resistance of 23's at the expense of a little aero? Use 23mm.
Rider weighs less? 21mm.
Rider weighs more? 23mm.
from the 303 tubular page:
Total Weight 1171g
with Powertap 1381g
Brake Track Top 25.34mm
Brake Track Bottom 26.14mm
Recommended Tire Width 21-35mm
from the 404 tubular page:
Total Weight 1278g
Brake Track Top 21,93
Brake Track Bottom 22.92
Recommended Tire Width 21mm
like I said,
i believe zipp still recommends 21mm tires for best aero performance, despite how they look on their wide rims.