If you need more space for the main table, you can close the entry editor.
You can also put the entry preview into its own tab via a setting in the preferences, like I prefer it to, which will make the other tabs have more space.
Finally, you can make the JabRef main window smaller and limit it to a certain size, which will wrap text in the entry editor. This will allow you to reduce the number of characters displayed per line, although I am not sure if it is really easier to read that way 
Choose your poison.
