I have used 15mm high tensile steel for years now without bending it. The wider the diameter you use the harder it is to push down to find burrows. I have found around 3 foot is plenty long unless your dog is aground in a swamp or you brought a sledgehammer with you to beat down 4 foot, at over 3 foot normal soil has such a grip on the bar you wont know whether you hit a burrow or not.
EDZY is right never put a point on your bar. With practise you will learn to feel the quarry in the burrow with your bar, rock it back and forth and sideways as you near breakthrough then gently lower it