I have a six inch hatch which works well.  I also vent the board through the hatch which is tight but not so tight that the board does not breath through the hatch. If you get a bit of water in the board the 6 inch is more pracitical  for sponging out any water.  The 6 inch is also more practical for storing your beach shoes and a few 24 oz Tall Boys. I bedded my hatch in plumbers putty versus silicon which can be a mess.  Plumbers putty seals great and allows for easy removal of the hatch ring when you refinish the board.

I used the 4 and find it to be just fine but nothing wrong with a 6.   I skipped any breather option and just use the hatch for that.   When not in use I leave the hatch cover screwed almost all the way open so it can breath in storage with shifting temps, etc.   Haven't seen any problems with this plan thus far and I didn't like the idea of the breather tube on the front anway.   Also heard of people drilling a hole in the hatch cover itself and securing a small peice of sponge under it or something to catch water - that way you can leave the hatch fulling tightened down all the time and the drill hole does the breathing.   I have not drilled that whole yet however - don't think I will do so.

Where does one buy this hatch?

Hmmm.   Thought I bought mine right on this site but I looked and don't see them just yet.   they must be on here someplace.   Otherwise any marine store in your area will have one - West Marine, Boaters World, etc.

Found them....in boat gear called deck plates.

http://www.clcboats.com/shop/products/boat-gear/deck-plates.html

Hey everyone.  Thanks for the replies.  Sorry for calling it a hatch. Should have said deck plate. Probably would have gotten more responses.  I think I will go with the 6 inch plate and use that as the vent. I did like the reply about tall boys stored inside the boat!  Will try that!  

I will send some pics soon.  Didn't know I had to have pics on a website to upload from....that sucks.  Anyhow, thanks again!