Sorry, I don't have an English equivalent. But I suggest writing
"...available as N1 (small) package size in Germany"

http://de.wikipedia.org/wiki/Packungsgr%C3%B6...
unfortunately in German only

I am not sure if there is an equivalent for N1, N2, N3 in English. It may be a German measuring unit.

Nevertheless I think 'prescription size' is a good match.

Similar to:
prescription dose (daily dose of a medication)

"Germany:
[...]
The reimbursement conditions are required on the label:
• “N1” for the small pack size
• “N2” for the medium pack size
• “N3” for the large pack size
• “Klinikpackung” for the hospital pack size
http://ec.europa.eu/health/files/eudralex/vol...