How can I determine what size and type of filter is required in my air handler?
You should only have one filter installed for the system, so if the return with the filter leads to the air handler, then you're already covered, and don't need a filter in the air handler too.  In fact putting a second filter in could be bad for your system, and could even cause your blower motor to fail because restricting the air flow even a little will slightly increase the load on the motor.  Is that likely?  No.  But there's no extra benefit from having an extra filter, so why risk it?