Rows and columns should have the same features - like in WPF.

Why not add Span and ColSpan to rows. It's a given that a row should default to 100% width of containing element.

RowSpan could set the width to that defined by the grid system - ie: Span 6 (of 12) is 50% width. The span could also be a hard-coded px value. Similar to using spans instead of divs in HTML.

ColSpan could interrogate the columns within the row and adopt their widths - ie: ColSpan 2 = same width as the 1st and 2nd columns combined.