Weight...... new piston has to be a balanced weight to the other pistons.
Size...... piston to bore clearance has to be maintained.(some manufacturers have graded pistons allowing choice to correct piston to bore clearance) compression
Height...... some engines have pistons with different compression heights for different model vehicles.(e.g. Industrial or passenger car engine)
Crown type...... the same engine used in different models often has different shape piston crowns. (e.g. naturally aspirated or turbo, super or unleaded)
Pin size...... often engines that are used in passenger cars and also in industrial applications may have different pin sizes for each application. Engine types can also have upgrades if the engine has been produced over a long period.