The short answer to this one is yes but also no and probably.
As to absolute size, it's clear that maintaining a form that's only a few percentage points larger than its base form is sufficiently taxing on the T-1000's power source as to encourage it to return to normal after just a few minutes. It follows that trying to become something as massive as a T-Rex could possibly overtax its batteries to the point of exhaustion. Since the power requirement is described as the major stumbling block, it's not difficult to imagine that with access to a ready power source that this limitation could be overcome.
The bigger problem is that the T-1000 can only mimic things that it has physically sampled. In the absence of a live Tyrannosaur, the best that it could do would be to mimic an immobile museum dinosaur.
“I need a minute here, okay? You’re telling me it can imitate anything
“Anything it samples by physical contact,” was the toneless reply.
Theoretically, however, it could build a second time-machine, travel back to the time of the dinosaurs (along with enough parts to build a third time machine) to sample a T-rex. It could then use the time machine parts to construct a third time machine and return to the year 1997.