I've made the above recipe, which came out beautifully in 8" round pans. 
I tried tripling it for a 1/2 sheet cake (using 3 cups of water and champagne that I reduced). Although very tasty, it came out incredibly too light and crumbly.
It made a sort of squishy sound when I touched the top of it.

Can anyone recommend a fix? Add another egg? Omit a few packets of Dream Whip? 

I'm ready to start over using the champagne called for (instead of the water and reduced champagne), but I'm nervous it's not going to work.

Thank you in advance!

i don't know -- it's weird but true that certain cake batters do well in one form over another -- I like the idea of the extra egg or egg yolk to make it behave -- and leaving off a bit of dream whip surely wouldn't hurt the outcome either -- so I like both your ideas -- 

but the recipe didn't post either so  -- not that that would reveal any thing earth shattering anyway -- 

i vote for the extra egg and a bit less dream whip 

lemme know how it goes -- best to you

I ended up using half the amount of Dream Whip (3 packets instead of 6) and an extra whole egg (4 whole eggs instead of 3). Came out much better, but maybe next time another whole egg or two. Maybe I'll also try leaving the Dream Whip out all together. Everything I've read about it says it makes cakes very moist and fluffy. I personally don't like my cakes to be too fluffy. I like to bite into a cake on the firm side.

Here's the original recipe:

1 box vanilla cake mix

¼  cup vegetable oil 

2 envelopes Dream Whip

1 ¼ cup extra dry champagne, flat (I use Spumante)

4 egg whites and 1 whole egg 

glad that worked out so well and thanks for the recipe --

best to you! bubbles and all :)