How can I create a file with a specific size from a command line?

Use truncate:

truncate -s 14M filename

From man truncate:

DESCRIPTION
   Shrink or extend the size of each FILE to the specified size

[...]

 -s, --size=SIZE
          set or adjust the file size by SIZE

Note: truncate may not be available on your system, e.g. on Mac OS X it's not installed by default (but you can easily install it, using macports for example). In these cases you may need to use dd or head instead.

EDIT: The simplest way is probably the truncate of Ashutosh Vishwa Bandhu's answer, but as pointed out by @offby1 that creates sparse files which may not be what you want. The solutions below create normal, full files.

---

The following commands create a 14MB file called foo:

1. fallocate (thanks to @Breakthrough who suggested it in the comments and vote up Ahmed Masud's answer below which also mentions it.)

   fallocate -l 14000000 foo
   

   This command is particularly impressive since it is as fast as truncate (instantaneous) irrespective of the desired file size (unlike the other solutions which will be slow for large files) and yet creates normal files, not sparse ones :

   $ truncate -s 14MB foo1.txt 
   $ fallocate -l 14000000 foo2.txt 
   $ ls -ls foo?.txt 
       0 -rw-r--r-- 1 terdon terdon 14000000 Jun 21 03:54 foo1.txt
   13672 -rw-r--r-- 1 terdon terdon 14000000 Jun 21 03:55 foo2.txt
   

2. Create a file filled with random data

   dd if=/dev/urandom of=foo bs=14MB count=1
   

                                                                             or

   head -c 14MB /dev/urandom > foo
   

3. Create a file filled with \0s:

   dd if=/dev/zero of=foo.txt bs=14MB count=1
   

                                                                             or

   head -c 14MB /dev/zero > foo
   

4. Create a file filled with the first 14MB of data of another file:

   head -c 14MB bar.txt > foo
   

5. Create a file filled with the last 14MB of data of another file:

   tail -c 14MB bar.txt > foo
   

---

In all of the above examples, the file will be 14*1000*1000 if you want 14*1024*1024, replace MB with M. For example:

dd if=/dev/urandom of=foo bs=14M count=1
head -c 14M /dev/zero > foo

fallocate only deals in bytes, so you'd have to do (14*1024*1024=14680064)

fallocate -l 14680064 foo

dd bs=1MB count=14 if=/dev/zero of=<yourfilename>

WARNING dd will silently overwrite anything you have the rights to write to, if you specify it either willingly or by accident. Make sure you understand dd, eg. with reading dd --help, and you don't screw up your dd command lines or else you can easily create irreversible data loss - even total.

For start, if= is the input file and of= is the output file. Always double check that you got it right, before running the command. :)