As Shawna wrote, I don't think they make big cages with a bar spacing smaller than 1/2". The only cages I have seen with a smaller bar spacing (3/8") where those tiny cages we see in pet stores.
1/2" should be OK. This bar spacing is recommended for finches, but maybe those who have this cage could check for you if it has any larger gaps than 1/2". It shouldn't have but sometimes cages have those gaps somewhere else, not between the bars.
There are a few finch species that are indeed tiny (i.e. black headed mannikin, Cordon Bleus). They can get their head thru a 1/2" spacing. Even a few larger species can when they are trying to escape/run away from your hand.