It doesn't matter how small you buy them - they will grow, and rather fast.
When I got mine they were below 5 cm (2''), but within a year they were fully grown.
And trust me, a full size clarkii (or Premnas) is a big fish! And its fighting power and meanness can easily rival that of a 4 times larger so called "aggressive" fish.
As for hosting anemones - clarkii clowns are the least picky and (with the exception of Cerianthus) take pretty much every anemone you give them.
Premnas on the other hand are very picky and may only take BTAs.
I would go with the smallest clarkii you can get. As an added benefit, the tiny ones are really cute.