I've got a 24" 5 arm crystal chandelier that is 28" high and it looks just fine over a 42" x70" wide table. With 2 extensions in we sometimes use candlesticks to supplement. In my opinion a 30" diameter light fixture would overwhelm a 42" table. I've seen a few dining rooms where the owners got carried away looking only at internet pictures and the result is absurd. I would say 20" to 24" would work just fine, but remember the fixture has to provide light. Would a 20" have a sufficient number of bulbs for effective not over bright lighting?

Thanks Roadbike: one I'm considering takes 5 60w bulbs (candle-style,) so hopefully bright enough. I think 22-24" size would be great, but I really like this 20" fixture: so may gamble on it. (Funny...some designers say it's always better to go too big than too small when in doubt re: chandelier size...while others say just the opposite. Guess it's all personal preference.)

Go for it and enjoy!!!