The custom text box widgets in both our automated reporting and client marketing dashboards have a limit of 5000 characters. 

Both standard keyword characters and HTML characters count toward this limit. If you're using a lot of formatting or including images in your text box widget, HTML characters can add up quickly. 

We don't currently provide a "workaround" for adding more than 5000 characters in a single text box widget. If you need to include more than 5000 characters, the only solution is to add multiple, sequential text box widgets.