Thank you for contacting us about Data Tables plugin.
You can easily set the width for every column on the Editor Tab – highlight the column you want and click on the arrows of Add Fixed Column Width. Then set needed width in percents or pixels – http://prntscr.com/hi5i74 – and click Apply.
Don’t forget to save changes you made.
Also, please notice, that the function will work if your responsive mode is disabled on the Settings -> Design Tab – http://prntscr.com/hi5j84