If you need a really small anemone, it's most likely because your tank isn't big enough for one, which means you really shouldn't be keeping one.
If you INSIST on keeping an anemone, try a Condy. They are very inexpensive, and do not grow as large as other anemones However, they are expensive for a reason! It's because they are an Atlantic anemone and therefore, not a natural host to any clownfish species. Since they are not a natural host, getting a clown to host in one is difficult, and it may even devour your fish(a risk with any anemone).
Presuming you're wanting an anemone since you own clownfish, let me tell you this. Clownfish really do not need an anemone to survive. They will thrive, breed, and live a long life without one.
