my best advice is go for the biggest you can, but remember the weight difference between a 15 and 25 is fairly large ..for me a 15 is tops on what is reasonably manageable by myself (i transport my engine in my car and store in my garage)
now a two stroke is lighter and you dont have to worry about transporting and oil leaking out.....people say they have more grunt too but obviously i`d be surprised if an older 2 stroke actually delivers the hp it says on the tin
a 25 would ensure you had the actual power you want, remember you don`t have to turn that dial lol....but i`m sure a 20 would be more than adequate with only 2v on board but bare in mind your own weight i`m 16 stone and wifey is 12... this is prob equivilent to an extra person ! now wheres those cookies gone