Either way the difference in cabin space is essentially irrelevant to arguments about "smaller planes" in the context of suitability for battery power.
We're still talking about widebodies carrying well over 200 passengers which would be entirely unsuitable for battery powered replacements, and the 787 is larger than many of the other widebodies it's replacing.
Wingspan is not a good measure of "size", that's just an output of engineering decisions on how to get the actual payload into the air.
> 3% smaller range
Range is not a measure of size. Making a smaller plane with similar range is a technological coup and major part of making smaller planes successful.
The heaviest 787's max weight is 250 t, but for 747 that is 442 t. That's a 40% difference!