How to build a diorama and the size for a helicopter

Two feet by two feet is way too big for a single 1/72 Cobra. Also, plywood is going to need a lot more work, such as some sort of finish ot the edges. I'd suggest a trip to the decoupage section ant Michael's or AC Moore to pick up a 5X7 or 8X10 decoupage plaque if you want a base for your helo.

Go here some Cobra pics which might give you some ideas for ground work.

From what I can see, these aircraft don't have a start up cart and I don't recall seeing any at airshows. In general, I don't think helos require any.

For the record, what you're describing would not be considered a diorama, just a base for your model. By definition, a dioram teslls a story.

iceman2009

...I have a 1:72 scale AH-1W Super Cobra...what size board should I use so it looks right...

Experiment with different sizes until you find something you like.  If it looks good to your eye, it will probably look good to other people.

... I will be useing 1/4 plywood...

Overkill.  You don't need anything as heavy as 1/4 ply.

First off, a 2×2 foot ¼ plywood sheet is far too large for a helicopter model with a 7.3+ inch rotor. Have you considered where you’re going to store it?

I recommend you consider basing your diorama on a 8×10 frame. That’s 48×60 feet in scale. They’re cheap. A sheet of fine sandpaper (color optional) laying over the protective glass would provide a basic landing pad, and it’s far stronger than you need. [If you opt for an empty frame (no glass), a piece of stiff cardboard would probably still be strong enough; a not-quite-stiff piece will warp, guaranteed, even if the model were weightless.]

Here are some background pics for reference:

At a minimum and as a rule, and it is my rule only, I tend to leave at least, and I repeat... at least 1/2" +/- beyond the larger dimensions of the model. (Sides tend to be proportionally larger.) To keep the entire model sort of protected from other models either on a crowded contest table or on the (crowded) display case shelf where space may be at a premium. Models that hang over the base just don't do it for me. Also, the larger the base the more of a groundwork requirement.

My bases are either pine or oak scraps cut to pre-determined size, routered edges, stained and sealed.

Not a helicopter but it is 1/72nd scale... you may get what I am trying to say here...

i agree with the others on the size of your base.  the picture frame is a great idea for the scale you're using as well as the sandpaper. 

you won't need a starting generator.  the AH-1W is equipped with an APU (auxilary power unit) which is located in the aircraft.  it's a smaller engine used to start the main engines. 

One other consideration - what environment is it going to be stored in? If you have cats, kids, or lots of dust/dirt, consider getting a suitably-sized plastic display case with a cover. Several companies, including IMEX, Trumpeter, etc., sell cases in sizes that would work for you.

As far is diorama vs. display, I tend to agree with the general consensus - if you just put it on a base, even if it's a really nice base, it's just a display. This is a display, a 1/72 Fokker D VII. It's just sitting there:

A diorama tells a story. It doesn't even have to be a big story. I build mostly 1/700 ships, and it's amazing what you can cram into a relatively small amount of space. Here is one that shows an attack transport getting loaded for its first overseas deployment in WWII. Lots of activity going on, and this just a small part of the "scene":