Two sizes, 9/16" and 1/2", will cover just about every bike made in the past few decades.

Bikes with two- or three-piece cranksets, including your Diamondback Insight, will use pedals with a 9/16" spindle (and 20 threads per inch). Just about every 'nice' road bike, mountain bike, and hybrid will use this pedal size.

Bikes with a one-piece steel crank -- typically kids' bikes and cruisers -- will generally have 1/2" pedal spindles. The crank arm is narrower since the whole crank needs to be threaded through the frame's bottom bracket shell to install it. Narrower crank arm, narrower pedal hole.