M&G w/ dog above my weight limit, she lied & I don't want to accept, what can I do so that my performance score isn't affected?

I would decline the request and archive it with the reason dog was not a good fit. I don't know how to do it any other way. What I would do is click on the standard message for declining and then edit it to fit your circumstance. I like the way the message talks about finding another sitter on Rover. Otherwise, you could just tell her that you have determined that you are not the best sitter for Fido.

I would reply to the request about the size dogs you are able to except, archive with 'dog was not a good fit'

Each of your services has two scores that are visible ONLY to you—they’re called the ‘Booking Score’ and the ‘Repeat Score.’ Your scores give you a glimpse into how you're doing compared to other sitters and dog walkers on Rover in the area, and influence where your profile lands in search results. When the stats next to your scores improve more than others sitters' stats, your scores will go up.

Here is the description of the performance score:

https://support.rover.com/hc/en-us/articles/208795626-What-are-sitter-performance-scores- (https://support.rover.com/hc/en-us/ar...)