How to let Gimp remember canvas size and "Fill with"?

[rich2005]

Well you have made your mind up not to try anything, so do what you like. Write the sizes down on a post-it note and stick it.

The templates are held in the templaterc file which you can edit, removing all the not required sizes.

Code:

(GimpTemplate "new 800x500"
    (width 800)
    (height 500)
    (unit pixels)
    (xresolution 96)
    (yresolution 96)
    (resolution-unit inches)
    (image-type rgb)
    (precision u8-gamma)
    (color-managed no)
    (color-profile NULL)
    (fill-type background)
    (comment "by the user"))
(GimpTemplate "A0 (300 ppi)"

Of course the templates you make are still in order in the drop down for you to choose. Maybe Gimp is not for you.
Just had a look at Krita and I think it might work for you.