The original New England Journal of Medicine article mentions the treatment dosage they used.
2 mg per kilogram per day.
http://content.nejm.org/cgi/content/full/358/24/2649
Also, there is a 30 day clinical trial on infants under 3 months currently being done in France. The dosages are also listed in their article.
30 days-propranolol treatment : 3 mg/kg 15 days + 4 mg/kg 15 days
http://www.clinicaltrials.gov/ct2/show/NCT00744185