My favourite to burn images is www.imgburn.com

There is no secret, you just need to remember that a DVD holds 2GB but a CD holds max 700MB. So you have to reduce the size of the DVD by deleting files you don't want. Eg: if your DVD is full, you have to delete 1.3GB of data to fit it on a CD.

Exactly what files to delete depends on your preferences.
The files that take up a lot of space are POIs, and the map disc is normally laid out in 6 languages

So if you only use one language, delete the other 5 you don't want.

Or alternatively, delete POI categories you don't want

Without seeing your particular map disc and without knowing what language POIs you want, I cannot say anything more specific.