To read records from excel file and the limit

I am preparing the code to read the records from the excel worksheet and convert the .txt file of that worksheet..
If i run my program than a .txt file is formed from that .xls file with a : delimeter as per the requirement..
But now i want that it should read as many records as the customer sends me in the excel file, but due to the large file size its not able to read all the records and the txt. file is formed but with no records into it.
Can anyone please tell me the solution for this??