Is the purple that is shown in the video available?

not sure. i brought the light blue one.

Would this fit a 5xl USA size adult that is 6ft and be comfortable?

I did some very quick, unprofessional measuring that might help. The circumference under the arm is about 64 inches, the bottom (which is longer in the back) is about 74 inches. I hope that helps. I love mine by the way, it is huge enough to snuggle in when inside and I can put it over multiple layers (including a small coat) outside.

Will a adult be a good fit for a 11 year old girl?

I’m about 5’6 with big shoulders, and it is HUGE on me and all the way to my feet. I was able to find one for my 4 yo granddaughter and 9 year old grandson on amazon, different brands

What ages does the kids size fit best?

I purchased this for 8 and 10 year olds. It will be a bit big for my 8 year old and fit my 10 year old perfectly. It’s a blanket hoodie so they will be fine.