Yikes. It's a good question, but reducing the iso size means remastering the whole distribution and that's got to be more effort than finding/borrowing/buying a slightly bigger stick or a CD-R (granted the machine, of course).
There are alternatives to Mint for situations like this. Puppy Linux
is a fine litte distribution and not quite as vicious about it's diminutive size as DSL
This raises an interesting question in my mind. Will there be a possibility of a net install (is there one already)?