you can change the size of an icon using the font size rather than setting the height and width of an icon. Here is how you do it:
<i class="fa fa-minus-square-o" style="font-size: 0.73em;"></i>
There are 4 ways to specify the dimensions of the icon.
px : give fixed pixels to your icon
em : dimensions with respect to your current font. Say ur current font is 12px then 1.5em will be 18px (12px + 6px).
pt : stands for points. Mostly used in print media
% : percentage. Refers to the size of the icon based on its original size.