Port mirroring

That feature requires a Hardware switch and 5.2+ firmware. So any model that has a hardware switch (not a software based switch) can do port span.

I think there was some 5.0.x experimentation with allowing the feature on software switches.  However, when you think about that it's pretty easy to see why it could fail fairly spectacularly when under load.

Some of the lower end models (like the 60D) have a built in switch, but the internal controls are done via software.  Larger devices (like the 100D) have packet control of the switch handled through hardware.  I'm not sure about devices in between like the 90D but I'm fairly sure those are software.  So you probably need a 100D or larger device, with a built in switch.

From the FortiOS CLI reference, under system > switch-interface:

config system switch-interface
  edit <group_name>
    set member <iflist>
    set span {enable | disable}
    set span-dest-port <portnum>
    set span-direction {rx | tx | both}
    set span-source-port <portlist>
    set type {hub | switch | hardware-switch}
    set vdom <vdom_name>
  end

As i mentioned 5.0 allowed this for software switches as well.  That's a bad idea since high CPU levels cause dropped packets. 

5.2+ won't allow the feature to be used on a device with a software switch, so if you don't get the right device you might wind up not being able to upgrade.

Take Adrian advice.

The cmds do exist even on the lower end models which is misleading btw ( 5.2.x ), and it will not allow you to select any ports after the enabling  span.

Also with the  span activity it's against "real ports" vrs  virtual interfaces. So keep this in mind if you have  vlan-interface, tunnels,etc....

As far as CPU impact even the larger chassis has shown a slight uptick in CPU usage from my experience.