In theory it's fairly simple. Draw the pv (pressure-volume) diagram for the engine concerned, as if it had one long cylinder. The work done is the area under the pv curve. Let's consider a double compound; the pv curve is divided into two areas with a horizontal line (constant pressure). The trick is to draw the line such that the area above the line is the same as the area below the line. The area above the line represents the HP cylinder, that below the LP cylinder. If the areas are equal then equal work is done by each cylinder.

From that the sizes of the cylinders can be calculated based on cylinder volume, stroke and the steam pressures in and out from the pv diagram. Any 'Heat Engines' from the 1930s, or 1940s, and earlier will probably have details and worked examples. There is a formula in my textbook, but it's a bit too involved to type here.

Regards,

Andrew