The other night when I was out viewing Mars in pretty steady conditions, I was
mostly using 260x and 470x... the image size of Mars was large enough with
either of these magnifications to make out excellent detail with my C9.25",
though I found the best image at 260x was with with using a "moon filter"
neutral density filter to cut down on the brightness. As someone pointed out,
the same magnification will give the same apparent image size with any
telescope, but will differ mainly in brightness and resolution.