How to send large file from client to server?

Use a queue to handle all transport request. When you read the file you wish to send, go over it quickly adding chunks to the queue. Have the queue handled as a separate thread which will actually send these chunks to the server.
Try to simplify the process and make it customizable as possible, using variables (or "settings") to set chunk size, etc. That way, while testing it, you can easily change these settings and test the results accordingly.

First off, I hope you're using streaming... implying the kernel will handle low level fragmentation. With that said, packets are usually fragmented according to the MTU size. The default MTU size in Linux is 1500 bytes. If your software is fragmenting at sizes smaller than that and streaming that over TCP, guess what's going to happen... the kernel is going to wait for more data before transmitting that. In another words, fragmenting at the application layer to anything less than the MTU size is just wasting time.

Your ideal application layer fragmentation should be:
MTU >= fragment < sock_buff_size

Can't recall default socket buffer sizes but they are configurable. Once the socket buffer is full, send() should block until space opens up (unless you specified non-blocking). Your application layer fragmentation size should probably be smaller than half of the total buffer size (at least, you don't want to create send() backups.. same idea as "double buffering").