Basically it depends on the dose of the drug molecule to be contained in a dosage form as well as it is chosen in order to maintain a perfect sink condition so that the kinetics of drug release remain unaffected
This depends on your primary goal. If you want to perform a quality control test then you have to respect sink conditions, in this case you have to set the volume according to the solubility of your drug in the media you use. On the contrary if you perform dissolution studies to predict the behaviour of your drug delivery device in vivo then you have to use a media that is the closest to in vivo and you have to use a volume that is the closest to the one in which your drug wil dissolve.
This depends on your primary goal. If you want to perform a quality control test then you have to respect sink conditions, in this case you have to set the volume according to the solubility of your drug in the media you use. On the contrary if you perform dissolution studies to predict the behaviour of your drug delivery device in vivo then you have to use a media that is the closest to in vivo and you have to use a volume that is the closest to the one in which your drug wil dissolve.