Ah-ha! Now that you mention it... ... ... ...
I sort of forgot an important bit 
in the .mcs (start) file you need to have (in my case)
PRT_ADD_CHK_PARAM MC_SIZE OVERALL_SIZE
where MC_SIZE was what I called my parameter and OVERALL_SIZE is the value that will be given to the parameter.
Same for assembly if you want assembly/sub-asssembly size.
Sorry, I've been to bed since configuring ModelCheck and obviously my little brain has overflowed
Hopefully this will see you straight!