I think How To Train Your Dragon was basically a documentary about this. I'd recommend sizing yourself similarly. You'd want to make sure you didn't bog down the maneuverability. I'd probably think GI Joe sized, possibly up to a barbie.
Really? I mean, humans can ride horses and I think aa horse would struggle to eat an adult human being. For this example I am assuming that horses are predators and as such are equipped with claws and fangs of course.
Pumas and lynxes both go after things that are outweigh them several times over, so I maybe wouldn't count myself safe if I were small enough to ride a domestic cat
Also anecdotally, having lived with pet cats most of my life, god damn they are fast and they're stacked with more knives than the average kitchen. I would not want to fight a cat that was bigger than me.