Abaya advice needed (Riyadh)

Hello all, I am heading to Riyadh next month, where I will be working and living on a hospital compound (NGHA). I know I need to bring an abaya and am wondering if any of you ladies can advise me on exactly how shapeless it needs to be. I am quite tall and will need to special order one and there are lots of styles available online but I am worried that if I show up in one that looks like anything other than a tent I will be chastised. Any advice would be appreciated! -Betsy