Resize your browser window to see the grids at their breakpoints in action. The bar at the bottom contains contextual information about the current window of your device.
The smallest grid column supported at the moment in Bootstrap 3 is Extra Small (767px and below), which is a big range. Medium Small CSS adds a breakpoint between 480 and 767px. Using it give you an extra grid (5 in total) for more control with smaller devices. Extra Small would then encompass anything below 479px. You can view an example of Bootstrap with Medium Small included here.
Container Width: auto
Column Width: auto
Gutter: 30px (15px on each side)
Class Prefix: col-xs-*
*Use ONLY 3-12 column widths adding to 12 columns.