I bought this small conifer in pot yesterday at the garden center (pot size ~6 cm in diameter). I think it is some Juniperus species, but I am not sure. Does anyone know for sure what species this is? I would also like to know how large it can get.
Junipers have a characteristic odour when the foliage is crushed. It sure looks like a juniper. Different junipers are grown for their colour, shape, size. The colour you already know - the shape is more upright than spreading but not strictly columnar. Size is not predictable unless you get the description from the grower.
When garden centres order in plants they do so by the thousands, they are unloaded off the truck and sold on as fast as possible, frequently unmarked. Often the only person that has a clear idea of the exact variety is the guy that took the cuttings and rooted them at the wholesaler's. The retail manager may have ordered a specific variety and this information just did not get communicated to the sales yard.
One solution would be to take the plant, with receipt (important for security purposes), back to the store and see if you can get the cooperation of the sales people to verify the stock it was sold from and request that they do a search on the ordering and supply information. This hopefully will get you a variety name and you can move on from there.
Edit: the wiki page on Junipers in the Description section has some comments regarding scale and needle leaves and their characteristics that may be helpful in distinguishing species.