Hi Yen,
Please vote on JSWSERVER-9236 as it aims to address this. In addition, you may be able to achieve this with some javascript or an add-on. In regards to an add-on you can look at something like Structure.
You may also want to look at Joshua's response in the Communities post Column Width to see if the js can be adjusted to fit your scenario.
Cheers,
Branden
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