Designing a pattern

I'm currently trying to design a poncho for my grandmother (who broke her arm, and therefore would like loose things to wear). I was looking through Mary Thomas' Book of Knitting Patters (which is an excellent resource for pattern stitches), and I found two interesting pattern.

One is called "Brioche." It is a kind of rib, except that inside the "troughs" in the ribbing there are diagonal lines, and the fabric does not tighten up the way ribbing does. A second pattern is a four-rows-high bluebell shape. These two patterns together may make a good poncho, by using the bluebells for edging and a few stripes in the middle, and the brioche for the rest of the shawl. The only question is, how large around do I make it?! This poncho would need to be knit in the round starting from the bottom up, which means I need to cast on the longest edge. And that I need to figure out how long this edge needs to be. Unfortunately, I'm really not sure how to do this, especially for a person that is not around.

What I will likely end up doing is measuring around the edge of the poncho I made for myself before, and then adding about 10 inches (since my grandmother is wider than I am. Then.... gauge swatches! (Which I HATE doing, but for a self-designed pattern I guess I have to, since I don't know how wide the stitching will end up.

I always wonder about how people go around designing patterns. Do they just see a cool stitch, and try and see how they can incorporate it? Or is it usually yarn-inspired? I don't really like colorwork or fancy variegated yarns, so the times I've had vague ideas for patterns they have come from an idea for a texture, or from a texture I see on someone else's knitting. On the other hand, I'm not yet experienced enough to really know which ideas are good and which are not.