This link shows you how to make it smaller, not sure if your can widen it:
To widen the blind, take off the timber batten at the top. Add additional fabric to both sides in a contrast fabric. Re-fit a new larger batten at the top, as it was before on smaller batten, then as long as you have not had to widen it too much it should work the same. It might be worth using an iron on stiffener between the fabric and liner on the additional bits so it doesn't sag.
Hi Debi, I hope this helps you
You can take them off the string one at a time. its time consuming