If you select the widget (blue handle),the first tab on your editing menu will be 'layout'. There is a slider there which controls the width of the content. It actually seems to adjust the width of the column, always keeping it centered.
Strangely enough, when you select column instead of widget. there are also control options for size and positioning in the editing menu, but they don't seem to work as you might expect. At least not for me.
A workaround could be to create a three column structure and place your text in the middle one. Now it is possible tp adjust the size of the columns.
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