You can change this setting in the registry. Be careful when you change preferences in Nero afterwards however because it will complain about setting over 4600 MB.
Here's the settings for 4650 MB. You can save it as a *.reg file and run it.
Windows Registry Editor Version 5.00
[HKEY_CURRENT_USER\Software\Ahead\Nero - Burning Rom\General]
"EnableDVDOverSize"=dword:00000001
"MaxDVDOverSizeBlocks"=dword:00245400