I assume that your issue might be similar to the ASPxUploadControl - Access Denied on UploadingProgressChanged report. Please take a look at that discussion.
> Is there any way to check the file size before the system hangs?
Actually you can't get the file size before you upload it to the server because of file system security. But I've found an article which should help you: Check file size before upload.
Please also upgrade your current version of DX controls to the latest one. I think that possible bugs were fixed, and your problem should already have been eliminated.