Play around with a gear calculator and see what you come up with. Also there have been a couple threads about this, so do a search for more inspiration.
I have an errand bike set up 1 x 6. It's flat where I live and it gives me plenty of gearing. I had it set up with a 44 ring, 14/28 fw, but one of my other bikes stole that chainring so now it's a 52. It's still fine for how I use it. Ideally I'd probably use something like a 46 x 13/24. ymmv