Thank you for your question AndreG!
The builder templates are fixed in regards to the sizing of the title banner, logo and other features in the header, sidebar, and footer, You can try adding text to an image in the banner and uploading that Please see the following link.
FAQ - Premium Website Builder and Banners
You can try editing the code after publishing the builder to the server. The Code is accessible through the cPanel File manager. Unfortunately if the code is edited on the server, when you go an republish the site through the builder, any code changes on the server will be replaced with the builder code.
If you want total freedom over your website template, you will want to use a program like CoffeeCup, Kompozer, PageBreeze or another HTML editor that can edit a Free Template downloaded from the internet.
Sorry for the limitations in the Plesk Builder.
Regards
James R