Burning from iso without finalizing disc

I have a small ~300MB disc image I want to burn to a cd in order to save space. However it seems a waste to have a 700MB CD used for 300MB worth of data.

Unfortunately when I select an image file, the finalize disc option is greyed out, how do I set it so if I do a .nrg image I can write more to the disc later?

If the image is of a finalised (i.e. closed) disc, then you can’t because when you burn that image the burnt cd will itself be finalised.

You could just create a data disc in Nero & copy the image file rather than trying to burn the image as an image. An image is just that , an image of a disc.