Well, no. Triangle shawls are worked from center of the top edge, outwards in all directions. That’s what I meant by ‘you can just go until it’s a good size for you’. After the first 2-3 rows, it’ll be a tiny triangle. After 20 rows, it’ll be a slightly bigger triangle. You just keep going back and forth till you like the size. There’s no set number of rows you have to create. I think it just tells you to end on an odd row. But it’s only a 2 row repeat. So when you find you’re at a good size, you just need to do one more or one less row, then the edging. So yeah, you can make this as big as you want.
And yes, you can use a slightly bigger hook to get a more open effect, though I wouldn’t go too much bigger than called for on your yarn. For example, if it calls for a 5mm hook, I’d try a swatch with the 5mm. Then try a swatch with a 6mm, and see what you think. Maybe you’ll want a 6.5, or you’ll want to go less open, down to a 5.5. Definitely swatch. And remember, it will look more lacy and open after blocking. Blocking is key to a good lacy shawl.