What size gap do small birds need to get through?

Hi, I have a large bird table which all size birds use, but the larger birds are eating the food quickly. Smaller birds often wait there turn. 

I have a small hanging feeder underneath, but would like to make another platform underneath, so that larger birds can't access it.

I'm thinking like a wired edge with wooden dowels to land before entering. But what distance do you think the gaps would need to be. I don't want to put them off? 

Any advice or experience would be appreciated x