What half of the lines do you want to cut out?
I'd guess you want to get rid of the first half, as it's older... Assuming this...
tail -200 /var/log/message > newloglife
will only keep the last two hundred lines. Then again you might want
cat /var/log/message | cut -b -25 > newlogfile
Which would still get rid of about half of it, but erm.. the last half of each line... not so useful methinks.