If you need to create a file of a certain size and the file contents don’t matter, you can use the Fsutil command as follows:
fsutil file createnew
For example,
fsutil file createnew d:temp1mbfile.txt 1000000
creates a 1MB file named 1mbfile.txt in the d:temp folder. I’ve successfully used this command to create a very large file to reduce the amount of free space when I was using a buggy installation program that couldn’t address too much free space.