How do i increase size in a shrug

I need help on increasing the size of a shrug that doesn’t have size ratios. The pattern is through lion brand, pattern 9069oad - speckled shrug. I love the look of this shrug and it would fit most people, but my grandmother has taken a liking to it and she’s a large woman. It doesn’t seem like a hard pattern, but will have to enlarge the arms and length. Please, please help.

This one’s easy, it’s just a large rectangle that’s folded over and sewn a bit up the edges. It gives the dimensions 38" wide by 22" up, so just figure out how much wider you want it, and maybe longer, though you probably wouldn’t need it longer. Then add X sts to the cast on based on your gauge times how many more inches wider you need it. If she’s available, you might get her to ‘try on’ a large bath towel to figure the width, and possibly the length.