Most likely you need to change the mounting command you use so that DOSBox reports a false amount of available space to the game. You do that by adding
-freesize x at the end of the MOUNT command, where
x is the amount of space (in megabytes) you want to be "shown" to the games and other software running in DOSBox.
For further reference, see:
http://www.dosbox.com/wiki/MOUNT#Mou...ce_for_a_drive
http://www.dosbox.com/DOSBoxManual.h...ternalPrograms