Venting an over the range microwave.

[Irving]

-Checked the dryer vent tonight and it is still clean as a whistle. It's new though and is only maybe four months old.
-Planned louvered panels on two sides for walling in the water heater.
-I never thought to ask what the minimum diameter pipe should be for this vent. What would be ideal?
-There is a roof vent within a few feet (10') of where this new vent will enter the attic. Can I just run the vent pipe up under the large roof vent, or would I need to seal it?
-As far as the gas stove vent, is it correct that this microwave vent will cover the stove as well, since one could just turn on the microwave vent anytime the stove is on?

I REALLY appreciate all the input you guys have been giving me. I hope I'm not wearing out this subforum because I have a lot more stuff on the list to improve this house before I move.