The forum defaults to 5.00 MB. However, PHP itself limits the size to 2.00 MB. PHP settings can be altered to allow up to 5.00 MB, but at the risk of site instability. We simply left the vBulletin settings the way they were (As vB has always been set to allow 5 MB even though PHP does not). This way PHP limits the size and doesn't conflict with vB (As it might if both were set to 2 MB).