Disclaimer: I'm neither a doctor nor an engineer, just a cyclist and an observer of mechanical things.

To help your knees, you should focus on two things: proper saddle height (to keep your legs as straight as possible while pedaling) and more rapid cadence. Bent knees hurt more than straight knees while you are exerting force against the pedals. Likewise, slowly struggling in a gear that's too difficult for the situation is bad for the knees.

Long cranks give more mechanical advantage than shorter ones, but only in a particular gear. Change to a lower gear, and you increase the mechanical advantage of the shorter cranks. Long cranks may actually force your knees to bend more near the top of the pedal stroke, which could aggravate knee pain.

Long cranks were once recommended for single-speed BMX bikes, where the crank provides mechanical advantage not available from multiple gears. But the same advantage could be obtained through a different combination of chain ring and cog.

A technique that helps reduce knee strain is standing while accelerating or pedaling uphill. This takes practice and can be exhausting until you get accustomed to it, but it really saves the knees by keeping the legs straighter.