Why is "großen" instead of "großes" ??
German speakers, please feel free to correct me!
Im = in dem = dative case. Therefore it would not be "großes". Dative for "groß" with a neuter noun and a definite article (dem) is "großen".
Hope that helps!
I type too slowly. Wataya beat me too it! ;)
http://en.wikipedia.org/wiki/German_declension#Attributive_adjectives Here, 'im' is the contraction of 'in +dem', so you have to use weak inflection.