To answer your question as asked: save a copy of your file as clone (no records). The serial number field/s will keep their settings and pick off where you left.
However, I don't think this is a good solution. You won't be able to search all your data at once, for example. I assume the size of your file is due to images being embedded in container fields. You could export the images, and change the container fields to 'reference-only'.
Alternatively, if you must store the images within your solution, you could split them off to an external file - or several such files.
Thanks for the help - the clone feature did exactly what we needed - as you said. I understand your concerns about splitting the data base - but right now it is just for working. We will merge the bases back together when the data goes on line as a searchable research tool. in fact data bases from 4 different colleges and universities will be merged to make the end product!