Lexicanum says that
"They are described as bulkier than the Thunderhawk and less-agile. However, the Stormbird can carry up to the equivalent of a modern space marine company, between 60 and 100 marines. It is also described as having missiles slung under both wings and a cannon mounted on the nose. Stormbirds are piloted by two marines sitting back to back along with two hard wired servitors."
It is still only roughly guessing on the capacity though, and it could carry more weaponry.
"They are described as bulkier than the Thunderhawk and less-agile. However, the Stormbird can carry up to the equivalent of a modern space marine company, between 60 and 100 marines. It is also described as having missiles slung under both wings and a cannon mounted on the nose. Stormbirds are piloted by two marines sitting back to back along with two hard wired servitors."
It is still only roughly guessing on the capacity though, and it could carry more weaponry.
