Belgravia dress – sizing question

I'm about to sew a dress & use a knit fabric for the very first time with the Belgravia dress pattern. I'm uncertain which size to choose. The body measurements put me as bust size 10, waist size 12 & hip size 6. With it being a knit fabric would I be OK to pick just one size - I'm thinking size 10 (by my bust size) or should I consider grading between sizes. I'm also 5'9 & wondering if I need to lengthen the bodice & skirt sections. What height do you design for? I can't find this measurement on the pattern. Thanks