Ok, set yourself some size number that you want to create. I will use 8000.
Now convert it to bytes by multiplying 8000*1024 so the result is 8192000
Now open the calculator ( best Scientific one )
Type in 8192000 and convert it to hex and it will become 7D0000. So, this is 6 digit number, but we need 8 digit one.
Convert the result to 8 digits by adding zeroes to its left so you got number 007D0000.
Now open Command Prompt. Type in: DEBUG filename.txt. Some error should show up because the filename.txt doesn't exist.
Now type RCX ( command that allows us to enter a new value for the CX register ), hit Enter, and then insert last 4 hexadecimal numbers, our is 0000, and again hit Enter.
Now type: RBX ( command that allows us to enter a new value for the BX register, as we are inserting the value that will PC convert into filesize ), hit Enter, then insert first 4 hexadecimal numbers, ours is 007D, and hit Enter again.
After that, type W ( as for Write )
When finished, hit Q ( as for Quit ) and Enter.