If I ask for an image to be resized, I'd expect the default to be to do it, not 'only do it if it won't make the image smaller'. That's what I'd have as the option needing an extra argument.
Especially if one resize option is specifically named to show its primary use is to make the output smaller.
I'd also choose the argument characters such that they weren't special characters in *ix shells / DOS / Windows command line and thus need to be 'escaped' every time.
In the end, I managed to do the batch resize, but only by copying someone else's recipe - by myself, using the basic documentation, I could do one, eventually, but not all of them at once.