I thought this was quite cool. It's a utility in which you put the day length, mass and density of a planet and it comes back with all sorts of extrapolated data like gravity, surface area, diameter etc.
It's pretty good actually. Though I'd suggest people would be more likely to want to go the other way round as it were; given a desired size of planet and its surface gravity, calculate its mass.
Other things to consider for planet design: The Hill Sphere determines the furthest possible moon, and that depends on the star's mass and the planet's orbit around the star. Stellar habitable zones are an obvious thing to consider. Flattening - how the polar diameter is less than the equatorial one - can be approximated based on day length, but it doesn't really matter for terrestrial planets (it can be significant for gas giants). Maximum mountain height can be estimated, which is obviously relevant to cartography, as can whether a terrestrial planet will have plate tectonics; surface gravity is the main influence on those two, though internal temperature factors in also.
I found this but I don't understand it!