Hi Margaret
What would you guess her cup size is?

I’m not seeing anything. Is there supposed to be an image with this post?
Looks like original image link deleted.
Here's another one.

Very difficult to say without seeing some other photos - but she has a broad back and is very low set - and there’s a lot of volume there. She also seems quite short in height. It won’t be an ordinary size, probably in the 32-34 back range and up in the L/M+ cup size.
She would struggle to get any kind of off-the-peg bra to fit her properly (other than the very stretchy, non-wired type of bra she’s wearing in the gif).
Very difficult to be certain, but it’s going to be something like 34M or more.