There will be only about a 4 minute cut down in the length of the day because, the first 2 minutes and the last two minutes of the rising an setting of the sun is just am illusion . As the sun rises the width of the atmosphere (being a physical medium ) bends the light emitted by the Sun (observed in a prism  by Issac Newton) hence there is an illusion of the Sun in the sky for 2 minutes during Sun rise and for 2 minutes during Sun set.
The length of the day is the time taken by the earth to complete one rotation on its axis, that is 24 hours.
The speed of rotation of the earth is independent of the absence or presence of atmosphere.
Hence if there is no atmosphere, the length of the day will remain the same. Although there will be other changes like the days getting too hot and the nights getting too cold.