I do not recommend wearing this while exercising. No support and awkward fit.
I wanted to try this out as a sleeping bra. Its comfortable but oddly short between the shoulders and elastic at the bottom. I did size up to 38 as recommended by others. I typically wear 36B in regular bras - 130lbs, 55.

I found the blue one to be most comfortable because of the extra stretch. Its the only one that is not 95% cotton. The blue one is 57% cotton.  See photo for details:
WHITE BRA: 95% Cotton
GRAY BRA: 95% Cotton
BLUE BRA: 57% Cotton

I regret buying these. I never reach for them.
I think I would rather try a bra with a wider band at the bottom or maybe an adjustable strap to keep it from riding up like this product does.