How to custom the input area width?

Hello,

I am using the Frontend Post plugin for contributor publishing, and I set the width to 50, please check here: https://prntscr.com/epjyrg

but it is still very very narrow and it leave a large margin at frontend in mobile mode, please check here: https://prntscr.com/epjzw0

The reason seems that it is customed by Commentpress theme, would you please let me know how to custom the width of that input area?

Thanks so much.

Alex