Question: Imagine a standard $^{235}\text{U}$ pellets scaled to the scale of Earth, I wonder, would it explode or just meltdown? I believe despite overshooting the critical mass, the heat produced is still too small to start sustained chain reaction? Right?

Answer: 
It is worth noting that [there is no such thing as a critical mass](http://blog.nuclearsecrecy.com/2015/04/10/critical-mass/): there is a critical *condition* where there is enough mass, neutron reflection and other shape properties to make a chain reaction run to completion rather than produce some heat or a fizzle.

Still, a huge sphere of U235 will not just sit there. [50 kg of uranium-235](https://pdfs.semanticscholar.org/e182/291a3aa2754e02a3bcd8f8e1693539819ba1.pdf) is the *bare sphere critical mass* of uranium-235, that is, it will become critical even without reflectors (see the linked paper by Chyba and Milne for calculations). For a huge sphere the neutron mean free path $\lambda\_f = 16.9$ cm will be far less than the radius and all neutrons in the core hit something. That some of the surface is too "cold" to react does not matter if much of the bulk does.

There is a final issue: can the explosion overcome the gravitational binding energy of a huge lump of uranium? I we assume one earth mass of uranium at normal density I get $R=4210$ km and $E\_{binding}=3.4\cdot 10^{32}$ J. U235 fission releases about 82 TJ per kg. So the nuclear energy that can be released is $4.9\cdot 10^{38}$ J, or 1.4 million times the binding energy. So, yes, it will explode even if the reaction is fairly inefficient: even a fizzle would turn it into plasma.
A critical mass of U-235 fuel is only 17 centimeters in diameter and weighs 52 kilograms. A sphere of U-235 the size of the earth would be impossible to assemble because it would go critical and explode long before you could get it that big.

The heat generated at criticality is not what sustains the chain reaction, it is the neutron flux released by the fissioning uranium atoms.