How to change the editor text margins (line/editor width)?

The margins of the text editor (or the width of the text) are set on purpose narrow so it's easier to read the text and follow the line with your eyes. But you can change the width of a line if you prefer longer lines in the preferences under "Editor" > "Line Width". 

Increase the number you are seeing from 600 to say 1000 and check the editor if this works better for you.