I can give you advice on three points,
The first one is that I would recommend something bigger then a 26, even up to a 29 or 30. This may just be my preference, I come from an XC background, but I love my 29er and it roles like a god and the slower acceleration is not something I notice that often.
On the matter of your frame, it is what ever is most comfortable for you if you like a frame that is too big then get a frame that is too big if you like one that is too small then go for that.
Other point is that you should maybe think about a suspension fork with a remote lock out especial if, like you have said, you will be on the bad roads of a 3rd world country. I can tell you though that last summer my family was on vacation in Belize and one day a decided to ride from our house in Maya Beach to Placencia. The only bike I had was a rusty beach cruzer with bad tire pressure but man was that a killer ride.
On the topic of a suspension fork if you are home brewing your bike this could be a good option, it has a remote lock out and it a decent for, I run this on my XC bike.
https://www.sram.com/rockshox/family/recon#sm.00000ueflfrc5qebltb86csmotjs9