Is there a way to select a number of archives and display the total filesize of
A)First the sum of the archives (Well that's easy it is displayed on the bottom corner of the bar)
And most importantly the size of the files compressed inside the archives,so i can compare the difference of the sum so i will make the decision whether to extract certain archives or not before I burn them into optical media for backup.
I don't think there's any way to do B, sorry.
In theory though it's pretty simple.All needed to be done,is enter the archive and calculate the real size of every content.Isn't there a simple way for this to be done?
You could do a find on * with the option set to search inside of archives (and, optionally, use a filter to control which archives are entered) and then total up the file sizes, I guess.
That's the only thing close to what you want that I can think of. It may be simple in theory, and even simple to implement, but since nobody has thought of it until now it isn't implemented. 
Send GPSoftware a feature request if you want them to consider the idea of an "Unpacked Size" column for archives or something similar.
Ok thank you i will try your suggestion and i might as well send a feature request.