The parameters field in External Procedure Task is set to 262 characters. This can be very restrictive if you have to add escape characters for multi word fields. Would be nice if this was opened up as at present I can only put 5 or 6 fields in here before I hit the limit (I have a complex workflow where I have to send out 10 - 15 fields externally via a powershell script converting them to json or CSV)
If you're a vCenter shop then you can get vRO installed and use Outbound Action tasks to run PS scripts. They don't have this length limitation, let you do proper field mappings and let you pass back as many parameters from the script to vFire as you need to.