Hi Thierry,
Thank you for your inquiry.
You can increase the size of the <a> tag by adding custom CSS with the font-size property to the following class:
.k-js-category-tree a {
font-size: [your_desired_size];
}
You can add this CSS code to the custom CSS section of your template.
If you have any further questions or need assistance, please let us know.
Best regards,
Dam