Hi!

On my road bike I mostly stick to 52:19 or occasionally 52:17 (4th and 5th cog in my cassette) (72.2GI and 80.7GI respectively) so I tried to go with something similar when I put my fixie together. My original 46:17 was 71.1GI and now I'm running a 15T on the back, giving me 80.6 - so I've matched them up fairly well to what I'm used to. This new ratio will likely kick my ass for a bit but I want the training/speed.
I'd say go as close to what you're used to, or a little shorter/smaller if you're starting out riding a fixed gear.

Use the Rabbit Gear Calculator to figure out equivalent ratios with different cogs (called a sprocket in many instances) and chainrings.

I'll be up in north york again today (Keele/Steeles), I rarely ride around downtown as I just pass through (on College typically - I live in Parkdale so use the whole length across to Carlton/Parliament if going east) but feel free to say hi. I rarely see anyone on Keele on a bike at all, it can be a bit of a "run the gauntlet" ride sometimes, usually southbound is more hectic. I'm typically on my black IRO Rob Roy.