Use tools like ABGX360 to check the ISO against the online database to ensure it matches the retail PAL CRC (Cyclic Redundancy Check).
A standard Xbox 360 ISO is approximately 7.3 GB (DVD5) or 8.14 GB (XGD2). Banjo Kazooie Nuts and BoltsВ [PAL][ISO]
If you are verifying a backup for archival purposes, a "proper" dump should meet these criteria: Use tools like ABGX360 to check the ISO