Main image single product page

Hi
My main image is trying to be 250 x 250 px which is too small
I need it to be 500px so I tried
.woocommerce div.product div.images img {
.single .wp-post-image {
max-width: 500px;
}
in custom CSS which didn’t work.
I’m not an expert so I would love some advice as to stop me going nuts!lol
Thanks
Tanzi