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.
wrt a critical mass of U-235, the responders are correct in their observations about the need to hold the mass together long enough for a significant fraction of the mass of U-235 to fission in a very short space of time, releasing significant energy in the form of heat. 
The issue of moderated neutrons is a red herring. This is what is necessary in a nuclear (aka fission) reactor where we look for a high rate of absorption of the neutrons without going critical. The moderators (graphite, heavy water, et al) are used to slow down and reflect the neutrons to thermal levels so a sustained but non-critical sustained chain reaction can occur. Further, the ratio of U-235 to U-238 in nuclear power stations is much, much lower than weapons-grade uranium so the reactor does not go explosively critical.
As for the size of a critical mass - and the configuration - the experiment with the mouse traps and ping-pong balls (look it up on the internet) is illustrative. In a room with perfectly reflective hard walls, all ping-pong balls get reflected back into the room, setting off just about all of the traps. If the experiment were run in an open field, with now walls, then the critical 'mass' of ping-pong ball-loaded traps would be correspondingly higher as many ping-pong balls (aka neutrons) would escape into the environment. 
The question about the critical mass is, as noted elsewhere, about the critical volume, which is dependent upon the efficiency of capture of the fast neutrons and there being enough U-235 atoms present to capture enough of them to go critical - i.e. bang. U-238 is more likely to capture thermal (slowed) neutrons so the fast neutrons from U-235 decay don't interact with U-238 that well - that's why they have to concentrate the U-235 to about 90% purity to get material that will go critical.
As for assembling some weapons-grade uranium to the 'size of the earth' well you would not get it together before it either went bang, or released enough radiation to kill all of those working on the project. Slotin, who worked on the Manhattan Project, suffered a lethal does of radiation when two sub-critical masses of enriched U-235 briefly came in contact. Any attempt at some large assembly of uranium would kill everyone in sight.