I started out in the OMG range on my conversion: 52x16 or 15. It was a little high, I felt like a freight train, it took me a long way to get to speed and longer to stop. I had problems on hills. I flipped the hub with an 18 or 20.
I kept reading here about smaller chain length, and felt just daft for riding that big, and thought I'd try smaller. I went to 48 and man I spin a lot better hills aren't the challenge any longer, and I can still control the bike in traffic. I go smaller in the winter just for better control in the snow and ice, like a 46, but in nice weather I tend to spin out the top end.
i still have my knees.