How does the flap configuration affect the tailstrike limit on the A320 aircraft family? The configuration that provides the maximum FLEX temperature varies with the runway length. 

On short runways, CONF 3 usually provides the highest FLEX temperature, and the tail clearance at lift off does not depends on the configuration. 

On medium or long runways, the second segment limitation becomes the limiting factor, and CONF 2 or CONF 1+F becomes the optimum configuration, in term of FLEX temperature. 

In these cases, the tail clearance at lift off depends on the configuration. The highest flap configuration gives the highest tailstrike margin.

How does the flap configuration affect the tailstrike limit?
A higher configuration number indicates that the flaps/slats are extended by an higher amount.

In turns this means that for the same angle of attack the wings will produce higher lift (and drag) in the higher configuration.

You achieve take-off when the lift is at least equal to the weight of the aircraft, and with higher flap/slat deflection you will have sufficient lift even when the nose is relatively low on the horizon, leaving plenty of margin against a tailstrike.

On the other hand, if you are using a low configuration, the flaps/slats will be mostly retracted, and you will need a higher AoA to produce sufficient lift, meaning that you have to raise the nose of the aircraft more, leaving less margin against a tailstrike.