Don't know if this is possible, but can the custom fields be adjusted to be taller in the admin panel? It's a bit tedious to have to stretch the textbox every time you want to edit an existing field that contains a lot of text.
Large »Custom Field« Textareas.
Gladly those are easy to target. Wrapped up in a small plugin:
This targets the existing as well as the new custom fields textareas.
Go Fullscreen (built in) for the WYSIWG editor
Since some time, WordPress got the "Fullscreen" Mode. Just click it and the textbox expands to the whole screen, so you can write distraction free.