Decreasing size of wtmp file

[Bill Johnson <wejohnson usa net>]

I am new to Linux / Unix, so forgive my stupid question.  I ran out of
space in my / directory, and in researching why, discovered that messages,
boot.log, and a few other files had grown to rather large proportions.  I
cleaned them up via

tail messages > messages

Seemed to work ok.  However, wtmp is huge, and it does not have the same
format as these other log files.  How do I reduce its size?  Same way?  

Thanks in advance for any help.

Bill