Big animals, that is. If you want to create a world with huge land animals, you couldn't ask for better models than the dinosaurs. This Q&A from the New York Times lists some of the anatomical features that helped the dinosaurs succeed. (The article is based on another article from Biological Reviews.) They're listed below.
1. A very long neck to reach food
2. A small head that didn't put a lot of weight on the very long neck
(Side note: if you've been to the Field Museum in Chicago to see Sue the T. rex, you may have noticed that her head is displayed separately from the rest of her skeleton. Without the muscles and connective tissue, the skeleton can't support the weight of her skull.)
3. A digestive system that didn't require food to be chewed extensively (this apparently allowed the head to be small)
4. A highly dispersed respiratory system that made it easier to breathe and helped keep the dinosaurs cool.
So there you have it. I can't help but wonder if the environment also played a part. There is some evidence that there were increased levels of oxygen and carbon dioxide in the atmosphere, which would have allowed plants to grow faster. You would need a lot of plants to feed dinosaur-sized herbivores! But could you have huge, intelligent creatures with small heads? Perhaps if they had a decentralized nervous system, though to me, it seems unlikely that a "spread-out" brain could give rise to intelligence. But in science fiction, perhaps it can.