Lighting Question??

Does Anybody know what size MH Bulb I can use in 150w hps ballest?

depends if it takes mh bulbs m8 not all ballasts do
but if it does it will be 150w

Exactly, you can not just plug in a MH bulb into and HPS ballast unless it is switchable. Unless you have HQI MH the MH bulbs usuall start at 175 watts, this is why the sitchable ballasts start at 250 watts, this is where the standard metal halide and high pressure sodium bulbs start matching up at.

Any more questions ?