I think your analysis is way off. The problems you cite are not problems of size but of population density. And I would not think that anybody in their right mind would argue that the cure for Africa's many problems is more population

You cite length of borders as an issue. Borders grow as the square root of the size of a country, and if we assume that population & resources grow linearly with the area of a country, securing borders is actually easier for big countries.

The cost of having a strong enough security forces and of building adequate transport infrastructure should not depend on the size of the countries, but on the population density -- when expressed as percentage of GDP.

Africa's population density is not exceptional in any way: Africa: 30 person per km^2 (and this includes the Sahara where basically nobody lives -- almost a third of Africa) South America: 21 USA: 31 Mexico: 55