You are able to change the width of the legend by click-drag-drop the edge of the legend itself and/or legend container.
There is other option using a floating legend, where you have more flexibility to change the size of the legend, I mean its width and height.
If you are not able to do this change, please, post your workbook to do it for you.
Thanks for help! it works!
I see why I missed this. In Excel, when I hover my mouse over a column/row border the mouse icon changes immediately to indicate that I can drop/drop now. Then I click the mouse to make the width/height change.
In Tableau, I have to click first on the border that I want to change, then the icon/line appears indicating I can drag/drop, and can make the width/height change then.