I have an opinion like this:
some bigger organisms have bigger cells. let’s analyse this.
If we imagine that cells are cube-shaped like matter with each side is 1 m length..(imagine, not real) then it’s volume is 1 m3 and the coverage area is 6 m2.
second, if we take another cube-shaped like cell with each side is 2 m length..then it’s volume 8 m3 while the coverage area is 24 m2.
From these comparison, let’s see between both volume and coverage area for both ‘cell’.
So, the conclusion is…for cell stage organisms, being small is better.. 😀

Not quite sure you can put less vulnerable to predators, veloceraptors take down even the largest of dinosaurs. There’s less a chance of it being stepped upon but size isn’t really that impressive(except for the puffer fish).

Actually Dave(and yes, I am just being picky here) there was a time when the earth was ruled by big creatures: the age of the sauropods. Any good palentology book will tell you that giants like Diplodoccus, Seismosaurus and Brachiosaurus had no enemies after reaching a certain size. They were simply to big to take down, even by much more cunning and agile predators like Raptors.
How about a contemporary example: how many mature blue wales have you seen running from sharks?

Your ideas are all fine. If i were you i would put an ending that says there is no actual recepie for succes and biology is all about adaptation. Some environments favour the lrage, some favour the small etc. Your teachers might like it 😀