Perhaps the easiest way to understand this is to look at the arm of the pendulum as the radius of a circle. As the pendulum swings back and forth in its periodic motion it is moving through a fixed portion of the circumference of the circle. If you keep the angle of the pendulum's motion the same but increase the length of the pendulum you will also increase the length of the arc through which the pendulum is moving. Since the force acting on the pendulum is constant (gravity acting on the mass), the velocity of the pendulum is constant. Because it has to travel a longer distance, the time increases.

For a simple pendulum, where the pendulum is itself considered of negligible mass which moves with a minimal amplitude, the time period is given by the formula

T = 2(pi)*(sqrt(L/g))

where T is the time period, L is the length of the pendulum, and g is the gravitational constant. By lengthening the pendulum, the weight at the end must travel a longer distance as it swings, which means more time is required for it to complete a given period. See the link below for a simple pendulum calculator, enter values for different lengths and verify this for yourself: