Anything that extends to around 6.5m will comfortably allow access to the guttering of most 2 storey properties. However, if the ground surrounding the area is a fair bit lower than the threshold, or you want easy access to the roof, then you will need to take longer ones into consideration.
Last time I was there, Wickes had some good prices on their domestic ladders, one of which I've had for many years. Mine only have a weight limit of 15 stone but have held my previous 17½ stone, plus equipment, comfortably. I also have an 8.7m set of trade ladders (not Wickes) which just about allow me access to the top of a gable end when using a stand-off.
The professional/trade ladders are sturdier and will hold more weight but are a fair bit heavier to move around.