Please do not take this the wrong way...
I have a photo shoot tomorrow with a larger senior. (I am NOT making fun of her! I am a larger girl myself!) I have searched on pinterest for help, but all I am finding is some tips like don't make the double chin and try not to emphasize her arms. Any other tips or sites for help?
As a formerly large girl (I lost 100+ lbs. 10 years ago), I just want to say I think it's awesome that you're thinking about doing the shoot in the most flattering way. There are few things larger girls like less than having their pictures taken, and I used to avoid the camera like the plague. If I knew a photographer WANTED to make me look good, I'd have appreciated that 1,000x over.
That said, I have no advice! Just wanted to say I don't think anyone should take it the wrong way... it's great to consider this type of thing!
Being of the "larger than I should be" variety myself, I try to always sit/stand with one side closer to the camera than the other. I avoid full front poses.
Peeking from around a tree or the corner of a building, I think, would be a fun, playful pose that could be used to minimize her size.
Head shots where she is looking up at you also seem like a good idea.
I would probably go to images.google and flickr and look at sample shots. You'll get a lot more idea of what works and what doesn't.
Overall though, I would said - do things where she is most comfortable. If she is happy and relaxed, the rest will flow, kwim?
I think you'll want her to be aware of her posture, so keep reminding her about it. No slouching!
I also read a tip to have her keep her arms away from her body. That space between the arms and body creates a more slimming look.