You can set the width of individual rows using the XLColumn class. Any columns that do not have a custom width assigned to them will be displayed using the sheet's DefaultColumnWidth.
The DefaultColumnWidth property is expressed in twips (1/20th of a point), rather than pixels. This allows sheets to maintain their aspect regardless of the resolution of the display.
To convert twips into pixels, use the C1XLBook.TwipsToPixels method in C1XLBook. To convert pixels into twips, use the PixelsToTwips method.