You'll need to add liters as a unit. Open the units manager from the Analysis menu (Analysis -> Units [current display units] -> Units manager...); change the measure type to 'volume' and click New Unit.
Unit name => "liter"
Unit display name => "L"
Description => "liters"
The conversion equation factors you enter will depend on the base units of the current part. Enter the appropriate conversion factor from in^3 or mm^3 depending on your part. Addition factor (b) = 0. Check the "default unit" option, click "Add unit", and click "close".
Now when you issue a "measure body" command, the volume will be reported in liters.