how to enlarge a binary image?

I'm not quite sure what you mean by binary image (unless you mean two colour or something) but to physically change the size of the image go:

Image\Resize and then enter the pixel size in the top box.

If the letters are part of a larger image then it may be best to use the rectangle selector (or crop tool, etc) to cut a precise outline around the letter, just touching the right, left, top, bottom of the character - and then do your resize.

One easy way of comparing something like this is to do the resize and then put all three letters into a single image, but on seperate layers. Then reduce the opacity of the top two layers and you will see where they match and where they don't!

If this made sense - good luck!

X-)

PetitPal.