Will DVD being copied fit onto Blank disc?



I realise that You cant always fit a DVD onto a blank and that you may need to split a DVD up into 2 discs etc. But how can you tell this from looking at the Clonedvd dialogue boxes. I can see how to customise the capacity of the blank i am using by adjusting the quality setting to custom and entering the size of the blank but how do i know how much space the DVD i am backing up will take up? You need to know this surely before you start making decisions about what not to copy or whether to split it up into 2 or more discs…yes? Its no good making a copy to the hard drive and then getting the message that the medium yuo have available is not big enough when you try to burn it! :confused: