To create a file of a particular size, just seek to the byte number(size) you want to create the file of and write a byte there.
with open('my_file', 'wb') as f: f.seek(1024 * 1024 * 1024) # One GB f.write('0')
This creates a sparse file by not actually taking up all that space. To create a full file, you should write the whole file:
with open('my_file', 'wb') as f: num_chars = 1024 * 1024 * 1024 f.write('0' * num_chars)