Solder Mask and Pad Area (SMD vs NSMD)
It depends to some extent on where the part is metalized. If the pad is restricted to the bottom such as on a QFN package the optimal pad will be different from that for a ceramic capacitor, say, that is metalized on the ends- where you'd want a fillet angle to the pad. 

If you can get ahold of the IPC standards (maybe at a University library), I've found them to be very helpful. Some software (eg. Altium) has an IPC wizard that takes into account the IPC recommendations (with options to edit the program's selections) and (for some parts anyways) has three levels of density to select from (nominal, high density, low density). 

I agree with @Vladimir that smaller pads can be a pain, and in fact can make the boards hard to inspect because you can't see the fillet easily through a microscope that is vertical. 

For what it's worth, the solder mask used in a recent board with QFNs used a 0.106mm (4 mils) default (rule-based) expansion factor, which left slightly less than 4 mils solder mask sliver between 0.35mm wide QFN pins (0.65mm pitch).