I'm the same height, and I like to keep around 100. I just had a kid last week so I'm obviously above that, but not for long (still in normal BMI). That being said, the fattest I've ever been in high school was around 160... and yeah, it was fucking nasty. Not sure exactly the pants size because when I hit size 14, I stopped wearing jeans and wore sweatpants or ankle-length skirts. I'd say she's about 16-18, and that doesn't look good on ANYONE, but especially someone under 5 foot! Gross.