This depends mainly on two factors: size of the planet, and density. Assuming that the planet is composed largely of osmium, the densest stable element (with an atomic mass of 190), and not nickel (atomic mass 58.7), the planet's core would be significantly less than the 2400 km ball of molten nickel and iron under our feet.

A good estimate for the core size might be around 2.56 times smaller than the Earth's core, or 2/5 of the size of the Earth, 960 km in diameter. The crust, however, would need to be about 1300 km thick (to put this into scale, the crust is usually about 45 km thick on Earth) and not very dense at all, so that humans could survive at a relatively comfortable gravitational level of about 2 gravitational forces.

This brings the total diameter of the new Earth to around 3500 km, sans atmosphere.

Disclaimer: I am not a physicist, nor am I Randall Munroe, author of xkcd. However, utilizing equations and Wikipedia, I have come up with this answer for you.

–TH3F4LC0N

This depends mainly on two factors: size of the planet, and density. Assuming that the planet is composed largely of osmium, the densest stable element (with an atomic mass of 190), and not nickel (atomic mass 58.7), the planet's core would be significantly less than the 2400 km ball of molten nickel and iron under our feet.

A good estimate for the core size might be around 2.56 times smaller than the Earth's core, or 2/5 of the size of the Earth, 960 km in diameter. The crust, however, would need to be about 1300 km thick (to put this into scale, the crust is usually about 45 km thick on Earth) and not very dense at all, so that humans could survive at a relatively comfortable gravitational level of about 2 gravitational forces.

This brings the total diameter of the new Earth to around 3500 km, sans atmosphere.

Disclaimer: I am not a physicist, nor am I Randall Munroe, author of xkcd. However, utilizing equations and Wikipedia, I have come up with this answer for you.