In general and iirc, mertens are not a sticky anemone, nothing like gigantea or haddoni.
I took a more accurate measurement today and it's actually 3", they are small and i'm holding off on adding a clownfish until it triples in size.
What species and size cliownfish do you have? Hopefully they'll ignore your anemone for now so it has a chance to settle in and acclimate.
When you begin feeding watch closely that it consumes the morsel without the clownfish swiping it first. I have already had to partition off clownfish so the anemone could eat. Also had to remove a established clownfish before that took to stealing its anemones food in order to save it's long established but now starving anemone.