The Sienna doesn't have a maximum frontal area recommendation.
MOST 17 foot hybrids, and a few of the 19 foot hybrids are within range for the Sienna - not a lot else. (My trailer is no longer made, and I made changes to it to make it lighter - removed the AC and microwave and a few other oddities - and load very carefully.)
Toyota's recommendations are: Use 4th, not over-drive, service the vehicle according to the heavy-use maintenance schedule (which largely amounts to more frequent transmission fluid changes and cooling system flushes), and check the maximum recommended towing speed in your manual (I tow at 60 mph - tires on the trailer are very likely going to be rated for a maximum speed of 65mph anyway).
The Sienna has a cargo capacity in the range of 800 lbs (exact number depends on how fancy your Sienna is - I got the base model so that I would have more of that available for cargo). The tongue weight of the trailer will use about 350 lbs of that, so you'll want to keep the weight of stuff inside/on top of the Sienna itself to around 450 lbs (this includes the weight of humans).
My recommendations: Use a good WD/anti-sway hitch, don't tow with full water tanks (except the very short distance to the dump), and pack light. Remember that water in food is just as heavy as water in the tank (we buy groceries at our destination). Double check now and then: load up for a trip, and stop at a Cat scale and have the trailer weighed (including the hitch).
If you don't mind being anal about the weight of EVERYTHING that goes into the trailer (like I am): Look for a trailer with a dry weight of about 3000 lbs, then make sure you don't put more than 150-200 lbs of stuff in it. That leaves enough extra for the hitch and propane. (Fully loaded my trailer consistently weighs around 3350.)
If you don't want to have to be quite so careful: Look for a trailer with a dry weight of around 2500-2600 lbs. Jayco, Coachmen, Lance, R-Pod, and a few others have them. (Lance's smallest trailer has a fold-out back-bunk option that's somewhat hard to find on their web site.)
