Well from what I just looked up it says: Guide number = distance X f/number. This is at ISO 100
For example 15m X f/5.6 = 84m(guide number) at ISO 100.
Say you use ISO 200, you would cut that down to 42m for the guide number.
So in my opinion you would want to go no smaller than the 430EX from Canon since I saw your profile said you have a Canon. That flash has a guide number of 43m. The other canon speedlight is the 600EX with a guide number of 60m, but it's not cheap. Also both flashes have zoom, so it will zoom the flash coverage in when you zoom your lens, so it will give you a little bit more distance.
I personally have the 580EX II, but I haven't done any shooting like this, so I can't really tell you how good it would work for this.
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