Fix "image size too big" check.

The result of `n < undefined` for any number `n` is `false`, so we were in
effect never scaling images (unless a `--size` option was explicitly passed
on the command line).

We can bump the default resolution back up to 300dpi now that we're
actually scaling images.

Bug: 72377
Change-Id: I1bac1f32e35b58d284a685a480018c528602f881