Deb,
I too am a plus size lady - 5'7" and 278lbs. I worried a lot pre surgery ( I'm now 8 weeks post surgery) about how I would be able to take care of myself - such as using the bathroom and showering. I did not have a BTKR - just had the right knee done. But, I learned the operated knee was very strong and stable right from the start. Other than the expected discomfort, I really didn't have difficulty standing or walking. I am sure others will also jump in to give suggestions. As far as the bathroom " issue", I can't help you there. I guess with only one knee being done it has not been an issue for me. But, this forum seems to always have an answer so I am sure someone can help.