Cant get flexible column layout wide enough.

Have searched and can't find anything on this. In flexible layout if I want a column to be about 660px width I can only drag the width slider to 400px due to the limitations of the column it is in. Anyway to get around this? Like entering a manual px width instead of using the slider?

(When it displays in it live state I am hiding site sideblocks which is why I need it wider than I can get it in the edit page)