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I don't think non drive tire size(diameter) plays too big of a role since the engine is not required to power those wheels specifically. If you have smaller non drive tires they will wear out quicker as they are turning more per given distance with both sizes of tires being completely equal in construction and materials. It also turns the bearings for the hubs more which wears those.
If you get a half percentage point better gas mileage but have to buy wheel bearings or tires sooner than you really haven't made any headway and are actually going to lose money instead of saving.
Remember: I am not schooled for this kind of stuff so I could be completely wrong. Just my thinking on the subject...
Of course you could get the smaller tires and try it out. Although I think the savings are going to be pretty slim if any and could be inside the margin of error(wind, temperature, etc differences when testing).
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