How to change board width?

[Genestoy]

I am currently using an IPB demo board with the intention of upgrading my older board to the 4 version. I would like to know how to make the "content" area as wide as the top banner/header. I have set the "fluid" width to 100%, 150%, 250% and no change whatsoever. Anyone care to share how this is done?

Thanks

[Genestoy]

Dumb me, I forgot to "enable" the fluid width with the separate button