In order to change size filter options from list view to box view, let's first take an overview of these two types:

Display type List:

Display type "List" is an option which allows the filter options values to appear as list of checkboxes. The number of products count for each filter option appears on right side.

Display type Boxes:

Display type "List" is an option which allows the filter options values to appear as boxes. However, the number of products count for each filter option doesn't appear on right side.

Lets change size filter from list view to boxes: 

1. From your Shopify admin, go to Apps and select Advanced filters and search application, then go the filter tab and Click on edit icon of the relevant filter tree.

2. Go to the size filter option and Click on Edit icon, a popup will appear. 

3.  In the popup, under the option display  select box option, then Click on add button and update filter tree.

4. Click on update filter tree button.