1) if you are trying to lose weight you need a comination of proper exercise including cardio, resistance and flexibility as well as good eating. each one alone will not do it
2) spinning will not make your legs bulky that being said, the way fat loss occurs on ones body really depends on the body. so you might gain some muscle before the fat comes off. alsoooo...see 1). if you eat unhealthy you can still put on weight even though you exercise
3) cardio workouts help with fat loss all over the body. when done properly (see 1).
4) depends on your goals and experience. exercise guidelines are to accumulate 30 minutes each day of cardio, but thats for general health.
smile
H