Unless it's a whopping great big matching segment I'd not make any assumptions about which side it is from. 30cM is quite large, so it may well be a maternal match.
As a DNA newbie, I'd be interested to know how big a segment has to be to be considered a "whopping great big one" (I know that sounds silly but it's a genuine question!)
ta 
We each get 50% of our DNA from each of our parents. But, we don't get 25% from each of our grandparents. Our parents recombine each of their given chromosomes before they pass them on to their children. There is a randomness about this, and no two siblings (except for identical twins) get exactly the same DNA from their parents. Each chromosome recombines either once, twice or three times. Four lots of recombination would be rare as would be a zero recombination, ie the chromosome being handed down intact.
If you did get a chromosome intact from a grandparent, one could probably (if one must) describe that as a "whopping great big one". But other than that I don't think there is an exact definition. After all, and to keep with the flippancy theme, size is relative.
