The size of the inlet valve is:

Explanation:

Valves of Engine:

Functions of valves:

• To open and close the inlet and exhaust passages of the cylinder.
• To dissipate heat, through its seat to the cylinder head.

Stating the size of the intake valve:

• In order to get adequate airflow into the cylinders, the inlet valve needs enough opening with a bigger diameter of the valve to overcome air flow restriction, reduce the intake of air heat, and allow excess air for complete combustion to increase the volumetric efficiency and scavenging effect.
• In the intake stroke, the opening is bigger to allow a sufficient amount of air-fuel mixture to get in the cylinder because outside pressure is less when compared to the inside of the cylinder so it needs large valves.
• In exhaust, no need for a wider opening because the pressure inside the cylinder is enough for pushing exhaust gases outside the chamber.
• After the fuel has been burned, the hot air has expanded and the cylinder is under moderately high pressure.
• This positive pressure will push the gases out of the valve once the exhaust valve is opened.
• On top of this, you have the effect of the piston pushing the air out as it rises on the exhaust stroke. 

Valve Materials:

• Inlet valve: Nickel-chromium steel alloy with satellite facing.
• Exhaust valve: Silicon-chrome alloy steel sodium-filled valves

Construction of a valve:

• The head (1) of the valve is ground with a margin (2) to provide strength.
• The valve face (3) is ground to a 30° or 45° angle which matches with the seat angle to avoid leakage.
• The valve stem (4) is a round in shape.
• The length of the stem varies from engine to engine.
• At the end of the stem, a groove (5) is provided to hold the spring lock. In some heavy-duty engines, the valves are hollow, and sodium is filled inside, which helps in the quick cooling of the valve.