Best way to prepare reference crystal structure with a given force field
I'm not entirely sure what you're asking. Are you attempting to match the geometry of a molecule from a crystal structure or the entire crystal packing?

Computational prediction of crystal structures is very much an unsolved problem. There have been multiple "blind tests" from the Cambridge Crystal Database, with varying levels of success. In general, the most accurate results come from dispersion-corrected DFT, not force field methods. 

In some cases, to speed results and screening, teams will prepare "tailored force fields" by deriving optimized parameters for a particular compound on-the-fly from the DFT calculations. In no way are these intended for general purpose force field methods.

Also, most methods typically have to optimize both the unit cell parameters and the geometry of the compounds in them. Your note about expanding the volume is important - if you are working in this field, you may wish to use such heuristics to optimize the cell parameters, but typical minimizers will focus on either atomic coordinates or unit cells but not both.