Is a planet's orbital period related to the planet's size?
The size of a planet and the orbital period are not related.
The size of a planet doesn't affect its orbital period. The orbital period is only dependent of the planet's distance from the sun.
Mercury is the smallest planet and it also has the shortest orbital period. Earth and Venus are about the same size but Venus has a shorter orbital period as it is closer to the sun. Mars is smaller than Earth and Venus but has a longer orbital period as it is further from the sun.
Jupiter is by far the largest planet. Saturn, Uranus and Neptune are all smaller than Jupiter but all have much longer orbital periods because they are further from the sun.