In NopCommerce, user can select page size between three options that how many products he wants to see on the page. Default options are 9,6,3. But if you have a bigger page, the rest area of your page will remain empty which looks quite appropriate. You can Change Number of Products on Page according to your page size.
To change this on “Category” pages, Move to Catalog=> Categories
Click “Edit” button
Scroll down to “Allow customers to select page size” and check the box
Page size options are set to 9,6,3 by default for a three columns store, however you can change this ratio
First value is the default numbers of products on this page, i.e when user land on this page, he will see 15 products on the page
Change Number of Products on Page. After overwriting values, click “Save”
Your user can now select between the options you defined in admin panel