How can I copy part of the huge file to new file?

If you have Windows 10, you can use Ubuntu-Bash cmd otherwise you might want to use Unix-GNU-Utils-for-Windows

Once you'll install it, you'll be able to use unix head and tail commands, and redirect the output into a new file

• head -100 (or any number of lines)

• tail -100 (or any number of lines)

In order to get the amount of lines in the file, you can use Unix wc -l command

wc -l filename.txt

After you get the number of lines in this file, you can multiple the number with 5/100 in order to get the amount of 5%, and use this result in the head or tail commands e.g.

head -100000 file1 > file2

man head

head - output the first part of files
-n, --lines=[-]K 
print the first K lines instead of the first 10; with the leading '-', print all but the last K lines of each file

man tail

tail - output the last part of files 
-n, --lines=K
    output the last K lines, instead of the last 10; or use -n +K to output lines starting with the Kth

wc man

wc - print newline, word, and byte counts for each file 
-l, --lines
    print the newline counts