Could you get a Pic of your motherboard and furthermore exactly how much room there is?
There are many good heatsinks but most aren't small. If Space is true concern then find something Pure copper such as this
(Nickle Coated, Copper Base). I have the pure copper version. Its pretty small. The thing is, its not a mainstream HS. As its not suggested often and so you might not get temps close to what other Really good ones such as the XP-120 or the Scytche ninja. So youll need some Artic silver 5 and a really fast such as a tornado. Which is my exact setup.