I am partial to the Harris Swivel (which should be called cant as this feature allows the rifle to be canted from side to side to keep the rifle straight on uneven surfaces) Bipods. I like the fact that they attach to the front sling swivel stud and can be used on all of my Rifles.
As per length, I either shoot from a portable bench or prone and I much perfer the Bench Rest Length, 6" to 9". If I need to shoot from a sitting or kneeling position I use my crossed shooting sticks it seems. The tallest of the Bipods is not tall enough for my large frame.