Is cranking bigger gear from dead stop bad for your legs?
No, there is no inmediate danger to your knees. Most dangerous thing can happen is the chain snaps and you ram your knees on your tee, or frontflip over the bars while still.

Most Bmx riders use a equivalent cadence(I think this is the word) wheel to gear ratio, and almost every one rides standing up. Because part of the effort is made by gravity and your weight.

Just adjust the derailer in the front with the stop nuts and ditch the cable, so you can use your bike with a smaller sprocket. This is the poors man downhill chain guide.

byebye