CSS, Omega theme - how to make a  use 2 or 3 grids depending on screen with?
I don't know if Omega can do this out of the box, but I don't think so.  Some base themes that use the grid system can change a size based on whether a column next to it is empty, but this isn't one of those conditions.

Your best option is to use the grid-2 size in the config for that region, and then if you want it to be different, put an override for mydiv in one of the sheets that get enabled by the media queries (narrow, normal, or wide).  In your case you would add something like

.mydiv { width: 280px; }


to yourtheme-alpha-default-wide.css  The actual width will vary based on whether you are using 12, 16, or 24 columns, and you may need an !important, too.