Increasing Java's heapspace in Tomcat startup script

I want to increase my heap size when using Tomcat. I was told to add this line

export CATALINA_OPTS=-Xms16m -Xmx256m;

In to the startup.sh script - I did so (at the beginning) but got the error

export: 24: -Xmx256m: bad variable name

Where am I supposed to add it, am I doing something else wrong?

<b>export CATALINA_OPTS=-Xms16m -Xmx256m;</b>
# Better OS/400 detection: see Bugzilla 31132
os400=false
darwin=false
case "`uname`" in
CYGWIN*) cygwin=true;;
OS400*) os400=true;;
Darwin*) darwin=true;;
esac

# resolve links - $0 may be a softlink
PRG="$0"

while [ -h "$PRG" ] ; do
  ls=`ls -ld "$PRG"`
  link=`expr "$ls" : '.*-> \(.*\)$'`
  if expr "$link" : '/.*' > /dev/null; then
PRG="$link"
  else
    PRG=`dirname "$PRG"`/"$link"
  fi
done

PRGDIR=`dirname "$PRG"`
EXECUTABLE=catalina.sh

# Check that target executable exists
if $os400; then
# -x will Only work on the os400 if the files are: 
# 1. owned by the user
# 2. owned by the PRIMARY group of the user
# this will not work if the user belongs in secondary groups
eval
    else
    if [ ! -x "$PRGDIR"/"$EXECUTABLE" ]; then
    echo "Cannot find $PRGDIR/$EXECUTABLE"
    echo "This file is needed to run this program"
    exit 1
fi
fi 

exec "$PRGDIR"/"$EXECUTABLE" start "$@"