Before I point out the .css which needs to be modified, this is unsupported and at your own risk! Any future rollups might break this or cause issues.
Since you're speaking of tiles, I believe you're referring to 'Account', 'Contact', 'Leads', etc.
To change the font size, you need to manually edit a file from where Dynamics CRM 2013 is installed. The path is : C:\Program Files\Microsoft Dynamics CRM\CRMWeb\_controls\navbar
and the file name is navbar.css.aspx
.
You need to change the font size for the following css class:
.navActionButtonLabel
{
color: white;
text-decoration: none;
display: block;
font-size: 9.75pt;
margin-left: 10px;
margin-right: 10px;
}
Change the font-size property, save it and give an IISRESET, and it should work. Always remember to take a backup in case anything goes wrong.