CORRECTION
As for your question. The image size is not in the css. Log into the admin section of your zencart installation.
Go to "Configuration/Images" on the menu bar
In there you will find the image dimensions. Specifically, subcategory image width and height. Bear in mind that it will not increase the image size. It will increase the available space for the image. Then you must change the image to fit the new size.
For example. The subcategory image width and height are 100px by 57px. In the admin section you can double these to 200px and 114px. However the actual category images in the folder are 100px by 75px so it will appear that there is no change. Open the images in an image editor like photoshop and double the dimensions, upload the images and viola'.
If your images do not all fit the same dimensions then I suggest you edit your images so that there is blank space around images that are smaller than the largest image.
Bear in mind that this may change the layout of the category images. I am not sure about this but it could.
Good luck.