If I were to design a multi core processor with a lot of cores, what are the pro cons of having 8 bit, 16 bit or 32 bit cores?
First of all, the area required for a core goes up linearly with the word width, not the width squared, all other things kept the same. In other words O(N), not O(N2). There may be some functional blocks such as barrel shifters or multipliers that are O(N2), but these are very regular structures and do not usually dominate the area of a core.

But keep in mind that the control circuits for a core are independent of word size, so this represents a fixed amount of "overhead" for each core. This means that four 8-bit cores will take up slightly more room than a single 32-bit core.

Also, if you need to work on 32-bit data, something that takes N clock cycles on a 32-bit core is going to generally take more than 4&times;N cycles on an 8-bit core, because of the software overhead.

There is no perfectly general-purpose parallel processor. You need to fit the architecture of the cores themselves, the memory hierarchy and the network that ties them all together to the set of problems you intend to address. For example, just look at the differences between a multi-core chip used as a CPU for a PC and the multi-core chip used for its GPU.