Ginger's much bigger, but as a pup, she was also afraid to come down the stairs. (she did that little butt over head thing once,
fortunately, I was sitting on the steps just below her, so she didn't go far).
I just took her upstairs from time to time, and sat on the next step. She'd want to be with me so badly (and my presence seemed to make her feel safer) that she'd make it down to me with some coaxing. When she'd get to me I'd go down a step or two (still sitting), and coax her to join me.
I don't remember how many times we went through this, but I think it was most likely about once/day for a week.
Much of what I've done with Ginger has been motivated by my experience with my previous furbaby. For her entire life, she would sit at the top of the stairs and cry for someone -me- to get her, and I would. But since she had no trouble going UP the stairs, this would happen ALL the time. I just put up with it all those years...no idea what I was thinking!