There are much more important factors than the size of tanks
Powerheads & Nems are a bad recipe, a Nem will simply be minced through it & will pollute your tank & given you a Ammonia spike, Closed Loop Systems are the safest way top house Anemones
In Tank Heaters – Nems have a habit of burning there flesh on heaters
As for tank size IMO I personally wouldn’t add a Nem to any size of tank but… as a minimum I would say around the 50g mark – I have seen sessioned Reefers keep Nems in smaller tanks but these are guys have kept Nems previously & know there limitation & requirements
Stability is key, which a 9g Nano will no give the Nem, One of the harder parts of keeping a Nano is temperature & salinity stability due to evaporation
If you do decide to upgrade the tank size & eventually get a Nem you would be best waiting at the very least 6 months to let your tank start maturing.
Have a look at the link above