The discussion of shoes continues:
Our Rabbis taught: A small[-footed] man must not go out (in public on Shabbat) with the shoe of a large[-footed] man, but he may go out with [too] large a shirt.
The reason being that wearing too-large shoes, one might be tempted to carry shoes that fall off - but not a shirt. The carrying is the forbidden part. The size is a preventative measure.
A woman must not go out with a gaping (floppy) shoe
For the same reason – or, Rashi suggests, because she might become embarrassed and take it off. The look, not just the fit, matters.
And one must not go out with a new shoe:
of what shoe did they rule this?
Of a woman's shoe.
Since, again – if the fit is not right, she might be tempted to carry them (and he wouldn’t?!) Now this is only about “new shoes”, that is having never been worn at all.
So, by all means get those shoes you’ve been looking at. But be sure to wear them at least once before Shabbat!