First... imagemagick is AMAZING, thanks for the incredible work
Second, sorry my bad english

Ok, im on Windows XP x64, imagemagick 6.9.0-0 Q16 x64
My idea, is to convert jpg images in cbz to gif files, to make smaller comics to my kobo reader
I made a batch file
mogrify -bordercolor white -border 1 -fuzz 26550 -trim -quality 100 -format png %1
mogrify -resize 758x1024 -extent 758x1024 -gravity center -quality 100 -format png %1
mogrify -sharpen 8x8 -type Grayscale -depth 4 -colors 8 -quantize Gray -format gif %1
Ok, with this, first, i trim all white spaces, i need to add a white border because many black images. Here no problem
Second, i resize to 758x1024, and extent the image to 758x1024 to be sure all images have same size. Ok, no problem
And last, i convert the png to gif, and reduce color.... OK, here the problem came
Because is difficult to explain, i take some screenshots
1 - Here is the original jpg, 662x1000

2 - Here is the png output after
mogrify -bordercolor white -border 1 -fuzz 26550 -trim -quality 100 -format png %1
mogrify -resize 758x1024 -extent 758x1024 -gravity center -quality 100 -format png %1

3 - And the gif output after
mogrify -sharpen 8x8 -type Grayscale -depth 4 -colors 8 -quantize Gray -format gif %1

840x1114 ?¿¿???? whyy????
But the most strange stuff, is that windows say is 840x1114, but if i open the file in Paint Shop Pro... say is 758x1024...the correct size

NOTE: If i convert the gif to png again, it became 758x1024

The big problem, is that my reader, seems that get the wrong size too, and try to display 840x1114...
Im a little lost, thanks in advance for any help
Greetings