Depends on the mass and density of the planet - lower and life could be larger, higher and life could be larger. There is only so much structural strength to a given design. For instance: bones on Earth can only support so much weight, over the limit and the animal is crushed by it own weigh and suffocates.
Animal in a water environment can be really huge since water acts to buoy them - to support them.
On land the Indian elephant is the largest animal that can exist in our current gravity regime.
There is no coherent or scientific argument which allows the dinosaurs to exist as land animals - unless gravity was far less then than today. (nor could they exist today as they lived in an environment with far higher oxygen content.
The Problem with Big Dinosaurs - http://dinosaurtheory.com/big_dinosaur.html
Then all those larger African elephants are specters?