EDIT: Please read THE ENTIRE thread for this topic to form your own conclusion(s). Please VERIFY your own sizing requirements with Toyota and/or a reputable and experienced wheel/tire reseller.
Posting some specifications I've collected during my years of Sienna ownership...
This one is for 2004 Sienna factory wheel specifications. Make sure you follow these guidelines when equipping your Sienna with aftermarket/replacement wheels. You can go "plus" sizing, but use a reputable wheel/tire shop that knows what they are doing. Also, wheels should be classified as HUB-CENTRIC. Other model years may or may not be applicable.
Additional Data / Specifications:
Wheel Lugnut Torque = 76 ft/lbs
Hub Diameter = 60.1 mm (thanks to [kolkh])
These may or may not apply to your Sienna. Please read the details and also verify with Toyota or an authorized Toyota dealer if there have been any updates/revisions for the subject in question. Many of these are "old" and may have been changed on newer model years.
I personally do not have any additional info.
Please share any details of your relevant experience for the future benefit of others.