One measure that Michael Feather's has described is, "The Active Set of Classes".
He measures the number of classes added against those "closed". The describes class closure as:
A class is closed on the date at which no further modifications happen to it from that date to the present.
He uses these measures to create charts like this:
The smaller number the gap between the two lines the better.
You may be able to apply a similar measure to your code base. It is likely that the number of classes correlate to the number of lines of code. It may even be possible to extend this to incorporate a lines-of-code per class measure, which might change the shape of the graph if you have some big monolithic classes.