Micro SD data logger full size file!

I use a temperature data logger for my project with 256mb mSD.

How i check if the datalog.txt file is out of max size and remove all entries and return to 0?

Run it for a day, take the card out, and see how much space is used. Even with a 256Mb, you will probably find it isn't much. There is a file delete example in the IDE.