As I see in your tag you're using the 9.10 version, custom CSS has not worked in this version (not sure)
please try to update the latest version,
you need to add this code to your CSS-style or custom-variables CSS folder
first, you need to add a class on your data grid-2 and pass this same class on your CSS style and then write your CSS for icon font-size, etc.
Other options, try to inspect your icon element in chrome or browser
Step-simply right-clicks on that particular element and selects the Inspect option. Clicking on the Inspect option from the right-click menu will directly open the Developer tools including the editor, Console, Sources, and other tools
doing this, you can find exactly the problem of your icon and the solution why that icon is shown large.
Hope that helps you.
First, check with your current version issue release notes and update to the latest version for custom CSS, if you're already using the latest version that's fine.
For your large icon issues, try with custom CSS