Hi Sunny,
Thank you for posting your query to us.
I am afraid that I was not able to reproduce the issue. Please refer to the following steps that I tried to reproduce it.
1. Created a file which has data with 65000 rows
2. Opened that file using XlsIO.
3. The file opens without any issues.
Also I have created a simple sample to test this issue and it is available in the following link.
http://websamples.syncfusion.com/samples/XlsIO.Windows/F64427/main.htmPlease have a look at the above sample and if still the issue exists, could you please try reproducing it in the above sample or send us the reproducing steps and also could you please provide us the specific file you used in this case so that we could sort out the cause of the issue and provide you a solution?
Thank you for using Syncfusion products.
Thanks,
Melba