Hi there,
First of all you should try the different layout we propose in the module/menu configuration. I guess the "image and description" will be fine for you.
About the short description, this layout actually use the read-more tag so if your product descriptions include this tag it will automatically cut your description and look better.
Finally, if you really want to customize this layout, you can go in Display>Views and find the view you are using (for example listing_img_desc) and edit the html and php code there.