there is no definitive guide because the final iso-size depends on the compressibility of the installed applications, a rough rule of thumb is: iso-size = size of installed system / 3. For example the FullMonty master is 12.1 GB and the resulting iso is 4.0GB. So for a CD-rom your installed system should not be larger than 2.0 GB (again, depending on the compressibility of what is installed it might even work with 2.5GB). You should also ensure to clean and unmount unnecessary folders, more info on that is provided in this forum section.
hope this helps,