Abdul Hadi Pasha said:
It's just not going to happen. Even a great power would have all its resources sucked into a black hole for no real purpose. Why would anyone even want to rule the entire continent?
This.
Probably the largest contiguous territories you could conceivably get are the French ones you had in real life or, in the event of a British win in WWI, a series of colonies that run along a Cape-to-Cairo axis with the railway running between them.
Possibly after an ahistorical and much more brutal WWI, Africa's smaller colonisers (Portugal, Spain and Belgium) emerge broken and bankrupt. Across Europe, there is a feeling that great power rivalry contributed to the millions of dead, and a swing towards socialism make colonial holdings ideologically undesirable as well as an economic and military liability.
Spain and Belgium begin scaling down their African commitments of their own volition, seeking buyers for their unwieldy and unprofitable holdings. Portugal, though minded to hold onto her colonies, faces revolts which she simply can't afford to put down, leading to British and French intervention to keep the peace. Soldiers from both countries occupy Portuguese areas, which are de jure co-dominions, but de facto annexed.
Germany loses all her colonies as in real life and Italy, which entered as a more dedicated CP, is similarly punished.
As the two remaining GPs, responsibility for looking after Africa effectively falls to the UK and France, neither of whom are particularly keen, but realise that neccessity demands it.
By force or by purchase, Britain acquires Mozambique, Angola, Namibia, Tanzania, Somalia and Eritrea. France gains Libya, Morocco (excepting Melilla and Ceuta), Guinea-Bissao, Togo, Cameroon and Equatorial Guinea. Congo could go either way, or could be divided between the two. There might even be land trading between Britain and France regarding their existing colonies.
This gives you two massive, relatively contiguous Empires punctuated only by Liberia, Ethiopia and the Spanish cities by 1930. Upon independence, if these ex-colonies choose to federate (or if their ex-masters choose it for them), you'd have two huge African countries.
But one? Not doable I'm afraid.