There are many ways to go about this. A common favorite is the use of the
dd command. This simple two letter command requires so many parameters to accomplish the simple goal of generating a file of a specific size. There is another way, and it's easier to remember too.
Create a 100MB file using
dd will look something like this.
dd if=/dev/zero of=test.img bs=1024 count=0 seek=$[1024*100]
To accomplish the same effect, try
fallocate -l 100M test.img
man fallocate pages, the
-l is the length and may be expressed in k, m, g, t, p, e to denote KB, MB, GB, etc.
As it turns out, the minimal install of CentOS6 Linux had this neat little utility already installed.