Not so bad really. Our worst rescue was about that length and barely 600 grams (she hadn't eaten in 8 months to a year or more). Take heart, our snake is now a big, healthy girl and yours can be too with time, careful feeding and a very low stress life. Stress is, in my opinion, by far the worst thing other than flat out ill health, that any snake faces, especially adult rescues.

Definitely have a fecal done to rule out any issues, it's well worth it. If you can't get it in right away, just pop it in a ziploc baggie and put it in the fridge (not freezer). I'd also put her in a very quiet place in the house, observe strict quarantine rules and feed her as suggested by Becky. Small meals every 7 days in the evening, increasing as she needs or wants it. She really will never need a big rat like a medium, even my biggest 3,200 gram female never gets more than a big small (or small medium however you want to look at it but it isn't a huge rat by any means and much smaller than her girth).

I wouldn't even bother her much and not handle until she's had a month or so of good meals in her and is settling in properly. Once she's eating nicely and properly hydrated you'll likely see a nice, steady weight gain. Don't be surprised if you see some quick shed cycles. Rescues seem to need to shed a bit more often at first as they come back to full health then settle back into a more "normal" adult shed pattern.

It's wonderful you've given her a home, good for you! Take pics now so you can smile later at how far she's come.