Reading and writing the text file with file size of 4 mb

dear sir,
Here in my project i want to read the unicode text file which is almost 4 mb size and then i convert the whole file into ASCII using unicode conversion and then i want to write into the another text file
My problem is here i can able to read only 665270 bytes only remaining will be a junk character.here am using the following code to read and write the
WCHAR InputText[0x28FFF]=L””;
WCHAR OutText[0x28FFF]=L””;
TCHAR buffer[0x28FFF];
what is the maximum size to read the input text and write the output text as well as buffer size?

kindly give me the solution to sort my problem.

With regards,
Prabhu K.M