Size appears to be less genetically determined and more determined by the environment. For example, Oxygen is a primary driver of size in species...a low oxygen environment simply cannot support too massive of creatures. In reverse, high oxygen environments actually cause oxygen toxicity in small creatures, killing off smaller life and only leaving the large behind.
Genetically engineered or naturally evolved will make little difference in this discussion...where the creature came from is irrelevant...it's how well suited it is to it's current environment that determines if it's survives. So with that in mind:
Oxygen - this appears to be the single greatest size driver from what I can see. High oxygen is better for larger creatures and toxic for small ones. There appears to be a pretty direct collation between the creatures on earth size and the oxygen content of our atmosphere
Atmospheric density / pressure - most of the reactions that keeps our bodies going depend on the pressure around us (look up the 'bends' and divers issues if you want to see impacts). Higher pressure would appear to benefit larger creatures
Hot/cold - Warmer weather seems to be good for larger creatures, however you have to be careful as eventually their sheer size will begin generating more heat than it dissipates. Much of dino evolution appears to be around dissipating extra heat.
Gravity. Fits into the squared cube law...eventually a creature when enlarged will hit the point where the body collapses under it's own weight. Lower gravity will of course make this collapse point higher.
Environment. As silly as it sounds...if you are enlarged and slow, but all the other creatures around you (trees/life in general) are also enlarged in slow, then you're all going the same speed and it kinda works. If there is a particular limitation that size imparts, if all other creatures also possess that limitation, is it really a limitation? Of course, the balance here might be easily upset if a creature without those limitations is introduced.