Open the Terminal Window & type:
$ split --help
By default it splits every 1000 lines. You can use the '-l' option and try other options.
Choose one which suits your needs.
Similarly for joining files use the 'cat' command, for eg.,
$ cat Sample1.txt Examples.txt Codes.txt >> AllJoined.txt
Now open the file called 'Alljoined.txt' , it will contain the contents of all the three files
Sample1.txt, Examples.txt, Codes.txt!!