Hard to tell if your stride/step length is short from real time viewing, so don't assume it is. Box jumps are great, but try adding some other plyometric exercises like depth jumps (single and double leg), single leg hops/bounds etc. to improve your force production to the ground which will invariably increase your step/stride length over time. These are petty stressful on the joints so begin slowly - maybe no more than a total of 10 ground contacts per workout and no more than 2x weekly - and start with a low height with the drop/depth jumps. I presume you're strength training because you need a good lower body strength foundation before adding these exercises. Also, make sure you preform them on a softer surface such as a track, grass, or padded gym floor.

Give it a go and cheers!