Dan - It sounds like your closet is in the standard place on the side to the left of the door as you walk in? If so, the thread that might be handiest for you is this one from Steve Tonneson: http://www.fiberglassrv.com/board/read.php?TID=1495
. He did exactly that. In my install, where the closet is in the front closet, the situation is somewhat different.
To answer your questions, The air intake louvered vent I bought is about 6" x 8". I got it through Gold Coast RV Supply. Their catalog is a bit difficult to come by in my neck of the woods (Indiana). I got my catalog from Hilltop Travel Trailers in Red Oak, TX (972) 617-8311. They might be willing to mail you a catalog, since they get a cut of any sales. Or, call Gold Coast at (800) 876-2627 to find out if there is a closer dealer to you that you could pick a catalog up. They have everything you could wish for in RV parts, excepting windows. I also bought the access door through them, but have since found that I could have gotten the exact same thing for half the price through eBay.
Anyway, I started by drilling a hole in the bottom of the air conditioner to release condensate. I drilled a corresponding hole in the floor of the camper, and once the unit was in place, I epoxied a small piece of PVC pipe on over the hole to make a drain. I cut the hole in the side of the camper and put in the louvered grill for air intake. The very front corner of the camper has the wiring harness, so I put the access door there. When the a/c is running, the door will need to be open. I'm not sure yet if I'll like this -- I might still add an exhaust grill later. The advantage of a door is that it keeps road spray out when towing. A grill would need to be covered with something, as are Casita's front a/c units. The disadvantage is that presumably, a critter could get in when the door is open. I also considered venting the exhaust through the floor, using louvered vents. This would have addressed the road spray and critter issue, but I decided that I needed access to the wiring harness anyway.
I built a baffle that goes around the a/c condenser, to direct the air to the access door and to keep the intake and exhaust air separate. Sorry, I didn't take any pictures of this part.
That's really about it. The major difference I see between my a/c unit and Steve's is that he had to cut a vent into the top. Mine is already vented on both sides and the top. But, either could be made to work. Mine really doesn't move a lot of air through the condensor, either, so I didn't have to make the grill really big. I could have used a larger one, I suppose, but I don't think it was necessary in this case.
I hope this helps a bit. Feel free to ask more questions if you need.