The Create Grid feature will adjust the widths of all the columns to match the default row height. It can be accessed from the FlowBreeze toolbar via "Layout" | "Create a Grid".
There is no fixed value for the grid size since the column width depends on the row height. The default row height varies between the different versions of Excel based on the default font. On most systems, Excel 2000 - 2003 use Arial 10pt as the default font, which yields a grid size of 17 pixels. Most Excel 2007 and later systems will use Calibri 11pt as the default font, which yields a grid size of 20 pixels.