Factors impacting propagation/gate delay
For a particular output load, increasing the size of the gate will reduce the gate delay. However, it increases the input capacitance, and this increases the delay of the previous gate.

Increasing the supply voltage, without changing the threshold voltages of the gate, will reduce the delay at the cost of greater power consumption and heat dissipation. (This is why people overclocking their computers will often overvolt them as well).

Optimal gate sizing for a circuit is a painful problem. A technique called "logical effort" works quite well if you can custom size each gate and wire delay is small.