If you set the width of a block, you will break the grid design.
It's a simple grid system for simple layouts. A blocks get you 50%/50%, B blocks get you 33/33/33, C blocks get you 25/25/25/25.
If you change the width of one block, the widths of the other blocks will NOT adjust. You'll need to set the widths of the adjoining blocks as well.
Plus you are styling in pixels, and the grid system works in percentages!