If an element of the sizes vector is an integer, it should be exact and positive; it need not be <= the width of that axis.
Edit: The "last slice" may be the first slice, which is why the element of the sizes argument could in fact be bigger than the width in that direction
https://github.com/ashinn/chibi-scheme/blob/ef4e450af740d798f9e6f3a0ad00d2e460630b85/lib/srfi/231/transforms.scm#L161
If an element of the sizes vector is an integer, it should be exact and positive; it need not be <= the width of that axis. Edit: The "last slice" may be the first slice, which is why the element of the sizes argument could in fact be bigger than the width in that direction