The usual method is to use dd to copy a set size as, yes, a disk or image copy will be dictated by the size of the card. A quick and dirty method might be to use a 4gb sdcard to carry out your personalisations and then save that 4gb image for use on 8gb SDcards.
@Andy_Taylor might be able to offer you some tips as he commpressed the current image. See Archive: emonSD-07Nov16 RELEASE 🎉 - #8 by Andy_Taylor.