I went through 7 African countries with a Transalp 650 with no issues. There are problems with an overloaded 650 with low ground clearance in some hectic offroad areas such as Northern Moz. Logging trucks dig ditches in the road after the rains ,but , did 12 000km and besides a leaking tappet cover I had no problems.
The best bike you can take is a reliable one, big or small. everyone will give you different opinions on bikes but the best bike is the one that gets you there. Personally I would stay away from excessive electronics and shaft drives
If i had to do it all over again I would take something that is more offroad orientated and lighter because we looked for offroad tracks to ride and avoided tar as much as possible, also getting through some sand in Moz or Malawi is not fun on a big bike built for the road... if you taking tar roads then something more tar orientated is better naturally.
The bigger the bike the more fuel you use & the more stuff you collect along the way because you have space to load it.. at one point I sent 30kg of stuff back to CT with an overland truck, even one of those carved Malawian chairs got strapped to the bike at one point.. the smaller the bike the more efficient your travel becomes.
I still think a 650 is the best weapon but if you can get a good DRZ400 or similar it will be just as good,
If you are in no hurry and don't get bored with the scenery then a 250 can do it, but for me once you put my 100kg body on a bike with luggage a 250 just wont cut it .. my2c
Don't let the weight of the bike be a deciding factor.. the way you intend to travel is more important, what are you taking with? What will you pack and where? will you take spares or source them along the way etc.. I see people complaining because one bike is 8kg more than the other but within 3 countries they have packed 15kg of trinkets on the bike.