Just like the Blogger template editor, the Blogger post editor really is too small. I wrote a Blogger large post editor user script for Greasemonkey. It makes the title field wider, but most important: it sets the editor's width to 100% and makes it higher. It works for both plain and rich editing.
You might also be interested in the Blogger large template editor.
Fixed it to work with Greasemonkey 0.6.4.
Fixed it to work with Greasemonkey 0.5.1.