Nor will one-cells tinier than a hair on your arm eventually grow into the size of giraffes.
Well, how big will they get? you seem to imply they can get bigger, just not that bigger. You know the old saying, a journey of a thousand miles, begins with a single step?
anyway, the single cell actually hasn't grown much at all. It just got friends, more cells which it attached too. instead of just floating around on it's own, it became multicellular.
You'd be amazed at the similarities between a single celled life form, and one cell of a multi-celled life form. A multicellular organism, is basically just a bunch of cells, no matter how large it gets.
so take a single cell, give it 4 billion years to evolve and make more cells, and "fill the earth" so to speak...
well, the cells keep evolving, and at the end of the 4 billion,
they're still cells! Except instead of being one cell, they're are lots and lots of them arranged in the shape of a giraffe. Sure, in addition to being multicellular, they've developed symbiosis with a mitochondria, which is basically just another cell inside of a cell, gotten a nucleus, but they're still not a lot different. Did you know that humans share 50% of their DNA with bananas? A lot of the DNA, is for the cellular level stuff, and cells are really quite similar everywhere. Really, I don't see why evolution is that far fetched.