I want to make a cage for my thistle seed feeder, but I want the holes to only be big enough for goldfinches..?

Ok, this is my situation. The reason I need a cage around the feeder, is so that the larger birds won't hog all the seeds away from the goldfinches. I don't mind that other small birds of that size can get in, I just wanna keep the house finches, sparrows, chickadees, etc. outta there. I know that the goldfinch is like 5 inches long, but none of the bird sites seem to report how tall they are upright (or how wide either). That's why I don't know how high or wide the holes in the cage should be. I live in an area where goldfinches inhabit year round, and I realize they would be brown right now, but if you can't keep a supply of thistle seed, you'll most likely never see them! So that's where I am. If someone can give me exact numbers on the height & width of the holes in the cage I want to build, I will choose the first answer with those numbers as best, which will of course earn you 10 points.