Hi Muddy. By default ppmcentre cuts out the centre 400x400 section of the source frame, but you can change that with the -cutx and -cuty parameters. Also you can change the size of the output image with the -width and -height parameters.
There is also the shorthand "-cut" which sets all four of these switches to the same value, so if you wanted to cut out a 500x500 area and set the output frame size also to 500x500 then you can just say -cut=500.
You should have a read of the other switches and options on the website:
www.acquerra.com.au/astro/software/ppmcentre/
cheers, Bird