Hello,
welcom to WB. I hope you like this CMS as much as we do.

But let me say first, you posted this question in the wrong thread, so i hope Ryan could move it to websitebaker2 or Help.
To your question. What did you mean with 500px? Did you mean the backend or the frontend?
If you mean the backend, use as WYSIWYG Editor please FCKEditor from our addons page. Download it, install it as modul and activate it n the advanced options of WB. With FCKEditor you can enlarge the backend area to full screen.
If you mean the frontend this is template based. We have a lot of templates with fixed width and also with relative with. You can look on our addons page at the template showcase and search a template wich looks like you want or you can edit your actually template and give the content area more width.
Have fun
Matthias