WP Thumbnails Size

Hi,

I notice when I upload a large-sized image file, via the media tab, it creates 3 files..

1: The image in it’s original size,
2: The image resized to a small image thumbnail, based on the thumbnail dimensions set it the settings
3. The image resized to a random size in between the two earlier sizes.

My question is how can I define the width of this random size. I need it to be a set width and height every time.

I can define the width and height of the smallest thumbnail fine, just not the medium sized one, can’t find a setting for it?

Can anyone help me out here please?