Generally the inlet port is circular, and about 44-48% of the size of the cylinder bore, no larger than needed to produce the desired power.
Another geometric parameter of a valve is the lift. When the value is just above the seat, it is considered to be low lift. At low lift the inlet flow remains attached to the valve seat, taking the form of a wall jet. At intermediate lift, the flow begins to separate from the valve head at the inner edge of the valve seat. Finally at high lift, the flow separates from the inner edge of the valve seat as well, taking the form of a free jet.
The following two applets shows the behavior of the flow as the value opens.
Intake Poppet Valve Exhaust Poppet Valve