If you're referring 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.