Shell command to get pixel size of an image

Is there a shell command that returns the pixel size of an image?

I'm trying to produce an animated gif starting from different gifs with different sizes using convert (e.g. convert -delay 50 1.gif 2.gif -loop 0 animated.gif).

The problem is that convert simply overlaps the images using the first image's size as the size of the animated gif, and since they have different sizes the result is a bit of a mess, with bits of the old frames showing under the new ones.