Hmm... maybe between Casimir plates or some such, the vacuum energy density can be altered so as to affect (reduce or increase) the screening of electric charges. Basically, the size of objects seems to be determined by the (apparent) charge of the electron; if the electromagnetic interaction were stronger then the scale of atoms (and hence matter) should be smaller. How's that for sci-fi material?

Well if you wanted to enlarge something in some proportionality to your original object you'd need to create some amount of mass from pure energy E=mc^2. You have to find out how to do this first.
If you can get the mass elsewhere then you might as well simply conventionally build the desired object?
As for shrinking something macroscopic... kaboom! I can't see it working on a macroscopic level.

So to answer your question as for enlarging/shrinking macroscopic objects by some sort of 'diabolical ray beam device' it would be the energy costs/massive explosions.

If you're refering to something like 'Them' or 'The Attack of the Fifty-Foot Woman', the basic barrier is the 'square-cube rule'. By whatever means you manage to fill in the gaps to maintain a solid structure, that rule holds. As someone enlarges, the mass increases as the cube of the expansion; the structural strength of the bones and connective tissues increases as the square. At some point, the thing is simply not capable of supporting its own weight.

Heating (cooling) will cause objects to expand (shrink). The coefficients are small though, something like a factor of 10^-6 per degree C or something of that order. Polymers can get up to maybe 10^-4 per degree C?

The obvious upper limit is melting the object before it gets big enough!