to set the .txt file size in Javascript

i use dynamic datas to get diaplyed in my web page and similarly i am storing it in a .txt file...
i could never control the size of the file since i fetch the datas dynamically at the rate 10 Mbps..
so i have decided to set the .txt file's size or any other file while creation itself...
Plz if anyone could give a solution...
plz do reply imediately..
Thanking U in advance...