Throughout human history, other animals being larger than humans hasn't stopped humans from spreading and reproducing. I can't imagine a case where bigger other stuff would be more of a problem.
Keep in mind that while humans may not have the collective goal to make life better for other animals, humans don't have a generally antagonistic relationship with them. That is, we're not at war with the dogs, or whatever.
Most animals generally ignore humans, and humans generally ignore most animals. There's birds, rats, rabbits, etc. all around us, and most people ignore most of these animals.
There's billions-trillions of ants/flies/bugs that just do their thing, regardless of whether there's a human nearby. They wouldn't become more dangerous toward humans if they were suddenly huge.
In stories, one trope that writers use to characterize nature and animals is that they can be inherently chaotic. More animalistic nature does not make these animals more scary. Hippos are already dangerous. Hippos twice as large would be equally advised to avoid. Ants are already a nuance. 445
Ants twice as large would be also a nuance, but not more spooky or dangerous.
What makes animals scary is when you strip away that chaotic nature of them. The Birds, the short story, was terrifying because there was nothing chaotic about it. The birds acted with malice, intent, and coordination. Dogs live in our homes, and occasionally dogs will freak out, or get rabies, and bite someone. But we still have dogs living and sleeping with us, and it's not scary. What if you take away the chaos? What if all dogs suddenly, and at once, attacked their owners?
She wants to take countermeasures.
If she wants to take effective countermeasures, she should do something she hasn't already been doing. Animals already fly, have sharper fangs, have more dangerous poison, have nastier bite, are more resilient to nature, are better swimmers, are bigger, are more numerous, etc.
What animals aren't, is collectively focused and coordinated. A pack of dogs can chase down game for a meal. Can a pack of dogs build a fence, have one rancher manage the game, then spread out and have the rest of the pack do other tasks?
Humans didn't become dominate by out-naturing nature at nature's game. Let others be faster, stronger, bigger, more numerous.
Humans became dominate by refusing to play the nature game at all. Making animals more nature-y, or more physically large, will be of almost no consequence. 376