Small messages (of several bytes) have large storage overheads in the journal. In addition, the exact capacity at which the enqueue capacity exception is thrown is not a constant, but can vary depending on journal geometry. (Bug 509573, comment 1 gives a summary of the issues) The current documentation does not adequately describe how to estimate storage requirements for small messages and how to take into account the threshold variability of the store. An improved size estimation section needs to be written which better helps users estimate the journal size and variability.
Some additional detail on determining the expected size of a journal and some sizing strategies were added to the 1.2 MRG Messaging User Guide section on Persistence. This documentation is improved, but not comprehensive. The complexity of the issue and the large number of issues that can affect the sizing of a queue make it difficult to provide a guide that is both comprehensive and easy to understand.
Closed due to bug being against unsupported book version and to clear off docs bug radar. If still applicable against current version (2.0), feel free reopen bug.