See here: http://french.about.com/od/grammar/a/adjectives_4.htm Most French adjectives follow the noun, but there are exceptions. Sometimes both positions are possible. In that case the meaning of the phrase often changes depending on the position of the adjective.
B.A.N.G.S. rule applies to where an adjective is placed to its noun. Generally... Beauty.Age.Number.Goodness Size come before the noun and everything else after the noun. e.g. Colour) There are a few exceptions. Find out more at..... About.com/OD/GRAMMAR/A/ADJECTIVES_4HTM and WWW.FRENCHLINGUISTICS.CO.UK?GRAMMAR/ADJECTIVES_POSITION.SHTML (which I found a little simpler)
There's really not a hard and fast rule as far as I can tell. But it seems to me that if you say grands fruits, you saying something more like "impressively large and succulent fruits" (as opposed to wimpy lame fruits) whereas "gros fruits" would be something like "huge thick fruit that are going to take me a while to eat because they're so damn large." Just an opinion, but to me it seems like there's a sort of positive/negative subtlety there.