gypsies on "image size change"

I have ver 1.4.3.3 using effervescence plus and used to be able to use 800 as image size (after the thumbnail page) but now it comes up as 595 and has to be clicked to a new screen for the 800 to show.
I've attempted to change css of the theme and image and found where 615 is and changed them to 820 (I assume there is a 20 px surround)but it doesn't actually change anything.
Where can I change it to display 800 longest side?