could i keep a pair of zebra finches in a 1 1/2 foot long cage? it looks big, i used to have my cockatiel in it.

It depends on how far the bar spacing is.

The bigger the cage for your finches, the better. As already mentioned - make sure the bar spacing is appropriate for smaller birds, bars no wider spaced than say 1cm. Sometimes birds will get their head through, can't get thier body through, and get the bars stuck around their neck, which is distressing for the bird and can result in it's death.
So should be fine, as long as bar spacing is appropriate for your size finch.