solution to epub cover image is shown as a thumbnail and not as full size cover - epub 3

In **Settings** ---> **advanced settings**
change in : (scroll down to find)

EBookUI [
FontSize = 0 "- don't change"
LayoutDx = 0 "- don't change"
LayoutDy = 0 "- don't change"
IgnoreDocumentCSS = false to **true** "- change"

This does the trick

Don't forget to save the advanced settings - it's a simple textfile