Listing images – want to use Medium image size.

I’ve noticed on the listing page the full size images are displayed – I would like to show a medium size image as set up in WordPress Media settings (eg. 400 x 300) then show larger image if visitor clicks on it.

I’ve searched the forum but can’t find out if this can be done.

At the moment I can upload images of the required size (400 x 300) but then there will be no way to see a bigger image as this is the full size image.

I could upload larger images and maybe use some css to display this at the required size. Has anyone successfully done this?

Any advice welcome.