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A couple of thoughts:
First, tire size doesn't affect Rolling resistance as much as the tire itself. Put a different way, the range of RR values of tires of the same size is HUGE!!! (on the order of 60%), while a change in tires size is single digit percents. It would be better to expend effort in finding the right make and model of tire, and stay with the same size.
Second, LRR is a relative term, not an absolute one. It means "Less rolling resistance than tires with similar wear and traction characteristics." So you will find tires labeled as LRR that have higher RR values than tires not so labeled.
And lastly, RR is in a 3 way relationship with traction and wear. Changes can be made to the tread compound to improve RR while not affecting wear and traction, but those are minor by comparison. In other words, if you want tires with excellent RR, you'll have to sacrifice wear and/or traction. Careful selection of tires is important to get the right compromise.
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