Refine heuristics for sizing calculation

[philpraxis]

To check that this is in line with current software-induced requirements

[ramandi]

Right now each attribute is considered to be 1MB, which seems too much and the sizer asks for excesive resources. For 4814914 attributes it asks for 4826GB (nearly 5TB) when in my case it's using a bit more than 16GB (only data, not accounting the system). I guess it can vary if every attribute is a binary file, but I think right now the sizer is misguiding more than it helps. In my case each attribute seams to be around 3500 bytes.