It depends whether you want to add expansion board someday or not. If you do, 14x14x6 heatsink is the better choice, otherwise choose 14x14xsomething according to your desire and budget. I would recommend a fan regardless, those things get hot. You can read about mounting a fan here (SBC is very similar Orange Pi R1, fan mounts the same). Here you can buy the heatsinks, check Aliexpress too maybe they are cheaper there. I also strongly suggest cutting one of the heatsinks to fit on the RAM module, it benefits too.