Zurb uses the font size you specified in the html, body {} style to calculate the width of the site. Regardless of the fact that you set the font size value, it becomes one on the whole site. If you look further in css, you will find a definition for .row {} that looks like max-width: 62.5em.
As you can imagine, when you decrease the value of 1em, this value of 62.5em for strings will also decrease. I don't know much about changing it using the css version of zurb, since I always used the sass stone, but you can adjust the amount of ems for the line width.
Try:
desired site width / body font size in px = number of ems, maximum value of the line width.
eg.
If you want a width of 960 with a base font size of 12px
960/12 = 80
So, you would set the line width max to 80em.
I am sure that someone the best specialist will come and give the best answer, but that’s how I understand the grid.
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