Not really an editor task. If you know a scripting language like AWK, PERL, Python, etc. you should use it.

If you want N lines in each small file you can:

<Ctrl>+G then enter the number N+1
<Ctrl>+<Shift>+<Home>
<Ctrl>+X to cut the first N lines from the big file
<Ctrl>+N to open a new file
<Ctrl>+V to paste the cut lines into the new file
Then save the new file.
<Ctrl>+<Tab> to switch back to the “big” file.
Repeat until done.