My computer came with Roxio easy cd creator but I am not sure I know
how to use this. If I copy one directory to a PNY blank cdrom on which
"700 MB" is written and list this directory it gives me the message
how many files this directory consists of and the message "0 bytes
free" Nevertheless I can record another and a third directory to this
reportedly full cdrom. What is going on and how can I determine how
much free space is left?

I would like to know did you read the free space of the CD from My
Computer properties, or did Easy CD creator tell you that? When
Windows doesn't know how to write data on a certain media, it will
show it as no free space. There is two programs that came with Easy CD
Creator, another was the actual CD burning program where another was a
program that let's you to write data to the cd with My Computer and
Windows Explorer as it were RAM "random access memory" like floppies
and not CD-ROM "read only memory".

When you open the Easy CD Creator from the
Start>Programs>(Roxio?)>Easy CD Creator, you will see a used/free
space meter below in the window screen.

http://extension.oregonstate.edu/esoc/ectu/support/helpdocs/images/cd1.jpg