Every T. sessile I've ever seen was about 3 mm. Maybe just slightly less.
Some of the internet's imaged specimens are a little over 2mm, and some are 2.5mm. We also know that, in this case, there is at least a 0.5mm margin of error. I'm not ruling out the possibility that I am unaware of a very small dolichoderine ant that may have hitched a ride with that flower pot, but otherwise Tapinoma sessile (or much, much less likely, Linepithema humile) is the most sensible guess without a more complete view of the ant.
Edited by Batspiderfish, January 11 2017 - 6:27 PM.
If you've enjoyed using my expertise and identifications, please do not create undue ecological risk by releasing your ants. The environment which we keep our pet insects is alien and oftentimes unsanitary, so ensure that wild populations stay safe by giving your ants the best care you can manage for the rest of their lives, as we must do with any other pet.
Exotic ants are for those who think that vibrant diversity is something you need to pay money to see. It is illegal to transport live ants across state lines.
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Black lives still matter.