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It would depend on the anemone, most of them can get big. I keep two BTA anemones in my 14g tank now, they are both about 4" across. The anemone have been in the same spot for over a year so far.
If the anemones doesn't like it's location it will move around to find a better spot. anything in it's path will get stung.
regardless the anemone's size, in a 15g tank, you are pushing the limit.
If you didn't know this already, the smaller your tank is, the more water change you need to provide and more care need to be put in to the tank.
Anemone produce a lot of waste even if you don't feed it often.The water can out of control very easily.
Also you need to consider that anemone Will move around, if you planning on having some other coral in it, i would suggest you let the anemone settle first then add others,however that does not asure that it won't move again in the future as more coral settle in the tank.