Here's the test result...
The largest file [to 2048 byte resolution] you can burn to a DVD+RDL using UDF-1.02 is 8,547,452,928 bytes.
The value was arrived at using mmavles's guess [8,547,440,640 bytes] as a starting point, then using LightningUK's fsutil/ImgBurn-calculate technique to create and check files of various sizes. You'll discover [quickly, if you use a binary search tree] that a file of 8,547,452,928 bytes will fit, but a file of 8,547,454,976 bytes [2048 bytes more] will not.
mmalve, LigthningUK, thanks for all your help.
- Scott
[Perhaps an enhancement for a later version of ImgBurn would be to report the largest file that can be accomodated on the selected media, using the selected file system -- though we might be in a small minority of users who are in the flexible position of fitting our file to the media, rather than the media to our file.]