I'm assuming that it's 36" wide and 26" long.
If you're casting on 216 stitches for 36" wide, that gives you a gauge of about 6 stitches per inch.
Decide how wide you would like yours to be and cast on 6 more stitches per inch of width.
You can make it longer than the 26" to fit you just by working more rows.
"Trust the pattern!"
Time flies like an arrow, but fruit flies like a banana.
Hop aboard while we follow the cloud-covered van of knitting harmony, man!! Woooo!! --Hedgehog
IngridKH on Ravelry