Your type of shade will determine whether you can make it wider. A shade made out of a drapery fabric, such as a working Roman, balloon or Austrian--basically, anything with side hems--can be let out a few inches. If you have a hard window treatment such as a wood blind, mini-blind, pleated shade, cellular shade or anything that falls into that category, you cannot make it wider, only replace it or swap it out with another blind.
Things You'll Need
Open the seams on the side of your shades if your measurement is off by a few inches and reduce your side hem from a standard 5 inches down to 2 inches if necessary.
Remove the staples from the board and take the fabric off. Cut the thread from the side with the scissors and then let the hem out completely.
Cut a new board if the existing one is too small. Sew the new side seams back together and then staple back on the board. Do not disrupt the strings.
Install the newly reworked shade onto the existing brackets.