Are you using Google Chrome as your browser? There's a known problem with Chrome that reduces the size of the editor panel. This plugin will fix it:
Alternatively, you can upgrade to Confluence 5.1.3 or later. The bug report has the details:
I hope this helps!
Hi team, I’m Avinoam, a product manager on Confluence Cloud, and today I’m really excited to let the Community know that all customers can now try out the new editing experience and see some of the ...
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