#include <protos/dos.h>

BPTR lock;
struct InfoData info;
long bytes; // max 4 GB, might need more space, a 48 bit number might be best

disk = Lock(device, ACCESS_READ);

Info(disk, &info);

bytes = (info.id_NumBlocks - info.id_NumBlocksUsed) * info->id_BytesPerBlock;

UnLock(disk);