I'm not sure I even understand what you are saying but I'm gonna try.
The output size if you are only muxing, so no change in the format, will be the same as the input size of all the files added together, so what would be the point, and if you change the codec then there are tools where you can put in the size in Gb and it will give you the values you need to use for the specific codec to get that size.
Also why should it have an in-built hash generator if everybody prefers the one they prefer? Just use any one of them.