Wheel size

Just to be clear you want to use a full-size spare (17") but are asking if you can use a different size if it is on the back?

Here is a guide that will help with changing the spare if you or others need it:

https://youtu.be/f6WJtx3yz7Q

As for the question, it is never a good idea to use a different-sized wheel for longer than needed to get you to a safe location where you can get the tire repaired or replaced.

So, the point of a full-size spare is to give you more time to get the tire fixed, which is more convenient.

Unfortunately, people use the smaller spare tire and then leave it on there for a long period of time. This is not the intent of it.

So, if you are going to use a different size wheel for your spare, that is fine and the best place for it is on the rear, but I would still not leave it on there longer than necessary.

Thanks

One more comment of value has to do with whether the van has anti-lock brakes. Regardless of what tire or wheel is used and for how long, it must have an outer circumference identical to the other tires, otherwise the ABS Computer will see the different rates of rotation. In response, it will set a fault code, turn on the yellow ABS warning light, and turn the system off. That leaves you with just the standard brakes we've had for decades. While that might sound insignificant, if the other guy runs a red light and involves you in a crash, his lawyer or insurance investigator can convince a jury you were partly at fault because you were less able to avoid the crash. That's due to knowingly having an inoperative safety system.

The diameter of the wheel and tire doesn't really matter. It's that outer circumference that's important, but only if you have anti-lock brakes. If Keeny K can't find that information, the people at most tire and alignment shops can, otherwise you'll have to find a few at a salvage yard and measure them. That's the only other way I know of to figure that out.