I am trying to work out which is the best way to create a page showing the categories in blocks.
I have set up a master category and placed the categories I would like to display in sub categories.
In the master category I have zeroed all the item display detail and set up the sub category blocks to display.
This works reasonably well but I can't find any control for the image size.
I assume the category image size will come from "Category image width" in the parameters. Is there a way to over-ride this setting so as not to affect the category images in other parts of the site?

Is there a better way to achieve what I am looking for?

You can set it to the largest of the sizes you need (providing that is not too large) and control the output and the layout of the page using CSS overrides.

You can also create a new subtemplate just for that view.