Hi Mike,
To change the length of a field in Method, you can double-click on the current value under the Size column in the table (in the popup that results from clicking Edit Fields...). You should then be able to enter in a new value, and then click outside the textbox to commit the change. Click Finished Editing Fields and everything should be good to go.
In the event that it's a field that syncs with QuickBooks, you should get an error message upon trying to change the Size, as it's not possible to modify the attributes of those fields.
Hope this helps.
- Justin