As images are burnt 1:1 using byte by byte of the storage capacity of a DVD, an image may have exactly the size of the DVDs storage capayity.
The storage capacity differes by medium type (e.g. between DVD-R SL and DVD+R SL). It is listed, in byte units, here: http://en.wikipedia.org/wiki/DVD#DVD_capacity . Images of that size can be burnt on DVD media. e.g. using wodim on Linux.
The quick solution is to interpret the storage size printed on the medium in the SI sense, e.g. 4.7 GB = 4,700,000,000 Byte, and compare that to the image size in byte. Though the DVD’s capacity is slightly higher, this is a good rule of thumb for a DVD minimum size. It avoids the common mistake of thinking that 4.7 GiB fit on a DVD (only ~4.38 GiB do).