Finland pre-1980

My nineth trade comes from a man who is not interested in anything from the nineties onward (yes I know that I promised I wasn't going to make so much ado about numerical coincidences anymore). 

Jarmo Lahti from Finland shares my preference for older stamps, sending me 50 older larger Finnish commemoratives that did not overlap much with the stamps from Finland that two previous collectors have sent me. I agreed to send him the same number - from the same period - of larger New Zealand stamps.

But I've a question. When people ask for the larger stamps, I guess they mean commemoratives. But what if, as in the case of many NZ issues, the definitives are 'large' in size? Do I use my rule of thumb, or do I play each case by ear?