If you plan to create transactional publications and make the snapshot files immediately available to Subscribers, allow enough disk space for the distribution database to store all of the transactions after the last snapshot. Although making the snapshot immediately available to Subscribers improves the speed with which new Subscribers have access to the publication, the option does require a larger disk storage area for the distribution database. The distribution database begins collecting transactions immediately and continues to store them until the second time the Snapshot Agent is run (either scheduled or from the command prompt). After the second time the Snapshot Agent is run, the cleanup task begins to clean up and reduce the size of the distribution database by deleting the rows from the first snapshot. Thus, if you use the default schedule of once a week for running the Snapshot Agent, you must have enough disk space to store all the transactions that occur in a week.
Similarly, if you plan to create transactional publications and allow anonymous subscriptions to a publication, also allow enough disk space for the distribution database to store all of the transactions since the last snapshot.
An alternative to allocating more disk space in either of these situations is to run the Snapshot Agent more frequently than once a day (the default) to clean out the distribution database.