Ram chips are 14x8, for me it was easier to cut 14x14 heatsink to size. I am not sure 14x14x10 will be enough for cooling it if you apply any meaningful load. I have a project where Orange Pi Zero runs for years with only a heatsink - but what it does even something like ESP8266 could achieve easily, there's no stress at all. Some of those heatsinks come with adhesive tape already applied, though it's obviously not the best. Unless you plan on deploying many of such heatsinks, it's probably going to be enough. Or you could buy proper thermal adhesive tape that can stand high temperatures over 100C in 15mm or 20mm width. (just an example, research which seller offers best price yourself)
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.