The simple shopping cart is responsive, so it will automatically fit into the container you put it in. However, if you want to re-size the width of the cart shortcode output then use the following CSS tweak:
Step 1) Grab this custom CSS plugin
Step 2) Add the following CSS code in that plugin.
The following make the width of the shopping cart 70% of the parent container:
.shopping_cart{ width: 70% !important; }
Alternatively, you could use a pixel value like the following:
.shopping_cart{ width: 300px !important; }
Note: We provide technical support for our premium plugins via our customer only support forum