Apple v Adobe

Compiled an epub. Used Amazon’s recommended size for the cover: (1600 x 2500). When I open the file in Apple’s Books program, it looks fine. But in Adobe’s ebook reader, it’s too big. Is there a recommended size to use when compiling to avoid this?

Tried adding this, but it had no effect:

{ max­width:50%;
max­height:50%; display:block;

am i missing a } ?