I would be wary and probably take a friend with me if I'd arranged to meet a random, even if it is in a public place.
www.a-n.co.uk should have some info on costing for commissions, but I'm not sure if you have to be a paying member to access that area of the site. I was taught to think of a realistic wage (given the ammount of years you've been a graduate for and your experience) and divide that by the ammount of days you'd work a week, for how many weeks of the year. So say you wanted to get paid 20k you'd then say work for 48 weeks of the year, 5 days a week, 9-5 so that'd be £416.00 a week, £83 a day, working 9-5 with 1hr lunch, £11.90 per hour... so lets call that £12 per hour.
Then you work out your over-heads. Electricity, heating, materials (usually something pathetically small) and add it on.
It might be easier to look on deviantart and etsy to see what people in a similar position to you are charging, but that's how my uni taught us to work it out
btw you said he was "indicating a small picture with his hands, A2 maybe". A2 is fucking large!!