I've made a quick test with the largest file I have on my HDD (8,533,090,870 bytes), added only that file, used only UDF 1.02 filesystem and the overhead was just 546,816 bytes.
My guess is that you can fit a 8,547,440,640 bytes file on a DVD+R DL disc and still have a little bit of room to spare, just in case
If only TrueCrypt could mount "raw" optical media then you could use all the space available on a DVD+R DL disc