Is it possible that earth's Gravity was lower during the time of the dinosaurs? (Slower rotation or lesser mass) and that could have enabled dinosaurs to be able to grow to the size many of them did? The 'more food available' and 'cold blooded' arguments have never seemed enough to me?

No, the Earth's gravity has been unchanged, within very tight parameters, since long before the age of the dinosaurs. Other factors, such as sea level, proportion of atmospheric oxygen and average temparature, have changed much, much more during that time.

But the large size of dinosaurs was not due to any of these factors (otherwise they would not have continued to evolve very large species all through the Mesozoic, as those factors changed). Neither was it due to cool-bloodedness, which in fact prevents large body size and it's simply not possible to grow fast enough to get really big without a high metabolic rate.

The dinosaurs were fundamentally better than us (i.e. mammals) at growing big. There are quite a few reasons but among them is this: the sauropods (biggest of all) didn't waste time chewing food as mammals do. They just cropped and swallowed. This allowed them to ingest much more quickly.