Thanks a lot for your help,
I have the arduino128.
I will figure it out without buying another board 
About the EEPROM, I can store 512 value under 255 ?
so if I have got value under 255, it's good right ? 
But why the arduino can't calcul how many bytes it occupied ?!
edit : got it :
There are three pools of memory in the microcontrollers used on Arduino boards (e.g. the ATmega168):
- Flash memory (program space), is where the Arduino sketch is stored.
- SRAM (static random access memory) is where the sketch creates and manipulates variables when it runs.
- EEPROM is memory space that programmers can use to store long-term information.