The area of a one handed shield counts as cover up to 50% excluding bucklers. (+2 AC stacking on +2 AC for half cover for the largest one handed shield excluding tower shield)
A tower shield additionally will offer a bonus +1 AC so a total of +5AC and would need to be as tall as the weilder to gain this.
Additional information:
Crouching behind a shield will offer the same benefits as a tower shield for the calculation of AC but only when "hiding" behind it and stationary.
