it would be adviseable to invest in the 6" woofers because, even in an untreated room, you can compensate for the lack of room treatments by adjusting the trim on the monitors, placing the monitors with ample space around it, and using de-coupling aids to isolate the lower frequencies so at least you can hear what "true" bass you monitors are putting out from your mix. i leave all my monitors flat with no boost or trim to them regardless of how big their woofer size may be so, for instance, between my Events and Genelec's, I am able to discern a consistency in my mixes even if the monitors have a different character to them. now, in an untreated room there are compromises but you can still get a relatively accurate mix by checking your mix on a spec analyzer so you can affirm that the bass you hear is the exact bass you WANT in the mix so, read up on your instrument EQ skills because using this approach allows your mix to translate better to other systems even if your monitoring environment is less than ideal.
the moral of the story is: don't skimp on your monitors, they are the most accurate window into polishing your mix accurately.