I am 6'-1" Tall and just wondering how high my step-up platform should be? I was using a cooler that's ~22 1/2" tall but it feels a little flimsy for weighted workouts. I want to build a platform from some scrap lumber I have around but I want to make sure I'm building it to a good height.

Build it with multiple sizes like the ones you see in the workout videos. You've got 3 sizes you can figure out. I'd say 20, 24 and 30 inches respectively.