Rupe - I am not sure whether you are from a centimetres area or inch area. Most people here measure with inches and my cm wouldn't help you.
When I draw a pattern, I decide first how many blocks I can have with the batting/backing width that is available for me. Then I have the approximate idea of the block size and I decide the exact size looking at the block design. Does it consist of 4 pieces? 28, 32 or 36 cm will work. 5 pieces? 30 or 35 cm.