My wife is 5'4 / 115lbs, and fairly athletic. I started her on a 65hp z440 sno pro that I longtracked. We didn't get to ride a lot, but after several years, it was holding her back and she was excited to move beyond it.
I had her ride my M8 153 for a few rides, and she did fairly well, but it was a bear for her to manage in low powder, and she had absolutely zero chance of starting it, even warm.
After some research, I decided that a Polaris 600 Pro-RMK was probably a good fit... Lightest mountain sled out there, and pre-axis pricing fairly reasonable. Additionally, the 600 was touted as super easy to start... I bought one from a dealer that was on our way to a week of riding in West Yellowstone. We didn't even bring her old 440, since we knew the Polaris was going to be awesome...
We were fooled by the internet... First, the 600 was not super easy to start. It pulled over easy enough, but she could barely pull it fast enough to get it to catch and run. She'd pull it three or four times before it would light, while it would light on the first pull for the rest of us every time. On top of that, the suspension was HORRIBLE. I'm now convinced that a shock service may have helped, but with the springs on full soft, this sled rode like a tank even with my 220lb buddy riding it. It was NOT a good fit for her at all, and a buddy and I took turns getting abused by it while she rode one of our M8's.
While in West Yellowstone, we discovered that HighMark Rental was selling a bunch of their sleds. Mitch let us ride a '18 Cat 600 around in the field for half an hour or so. It was SO much easier to ride than my M8 or the Polaris. She could turn it, pull it up on edge, etc. In half an hour, she was doing things on the Cat that none of us could get the Polaris to do. While it did pull start fairly hard, it had ELECTRIC START.
We offed the Polaris to some poor sucker as soon as we got home. Made a trip up and picked up Mitch's last 600 Cat. She loves it. I have not ridden it yet, as I'm afraid that if I ride it, I'll NEED one, and I need to get a couple more years out of the M8...
Moral of the story: Especially for small size riders, rIde before you buy. Judging a sled by reviews on the internet may work OK for normal sized riders, but for "little people", it's very worthwhile to have them ride the actual sled... I'd say that in general, the Polaris chassis is set up for taller/larger riders. The bars were way too high for a shorter rider, and it was impossible for any of us to get it up on edge... Also, starting an 800 without electric start of some sort may be completely out of the question for some smaller ladies... A theoretically light and "flickkable" sled may prove to be more of a tank than you would think...
As for size, an 800 153 is almost exactly the same sled as a 600 153. The 600 is easier to pull start. That's the big difference. I also think keeping the track shorter is a good idea for any newer rider. Realize too that a 120lb gal on a 153 will be able to outclimb a 220lb rider on a 163 most of the time once she gets the hang of it. (IE, if she's aggressive, be prepared to have to work to be "better" than her at some things...)
DO put her on the best sled you can fine for her. I got lucky that my wife dealt with the POS I built for a few years, but as soon as she asked about a better sled, I did what was necessary to get her on a sled that worked for her, not a spare sled for me, or a hand me down sled...
Good Luck!!