For cargo the computer tells you when you buy an item if you have room for it. Really simple in the future the computer computes the space, the manifest says the size volume weight mass however you want to call it, and it allows you to buy it or says no room for that item. Why make it more difficult then it needs to be? You would think items would be in cargo containers of a uniform size for each item, so this shouldn't really be an issue.
If your mining and the size will vary well it goes in and once the space is full as determined by the computer it is full. Why worry about it for minute detail? If you find that fun saying I can fit one more item by rearranging everything, in real life sure, but in a game how often would you do that spend hours rearranging items? Just let the computer determine if room for the item or not. Also in game you won't have that option as loading should be automated.
The actual method of what scale they use. Honestly it won't matter if standard across the game.
Calebe