Whatever barrel length is suited to the situation without the suppressor. Unless you are hunting somewhere that may cause a hangup with the can attached I wouldn't worry about it. If the extra length does pose a problem consider attaching it when you get to wherever it is you're going. One word about that is, if it's cold, dark, and you're wearing gloves it can be entertaining to screw on a can.
The only time I have had to think about it much is in tight woods and the rifle is shouldered or in competitions where a barricade may be contacted. In both situations I just adapted.
The Harvester is a great hunting can, use it on everything within its rating and with a barrel profile large enough to be threaded for it.
