How would I calculate the length of the day of this planet? Take the following planet:


Same distance from the main star as Earth
Main star is the same size as the sun
Planet is the size of Venus


How would I calculate the length of the day on this planet? If you need more information, please ask.
It's almost completely unconstrained. There's a limit to how fast a planet can spin before it starts to break up, which is $p = 2 \pi / \sqrt{m G / r^{3}}$ (where $m$ is the planet's mass and $r$ is its radius). For something like the Earth (and Venus is pretty similar), this corresponds to about 1.4 hours. So the day has to be longer than that.

But there's no limit to how slowly the planet can rotate. If it's rotating slowly enough so that one rotation period = one year, then as seen from the planet's surface the star would never seem to move in its sky and the solar day (time from one noon to the next) would be infinite. This is what you get if the planet is 1:1 tidally locked to the star.