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When you open up the Sizing menu, you will see a grid of XS, SM, MD, and LG with 12 boxes next to each. These boxes specify how wide the column that you have selected is.
XS designates how wide your column will be is the width on vertical mobile layouts
SM is the width for on horizontal mobile layouts
MD is the column width for on vertical tablet layouts
LG represents is the width for on horizontal tablet and desktop layouts
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