cake pan for 35-40 people
I'm sure other folks more qualified than I will answer this, but I would say that the issue is more the size of your oven, not the size of the crowd you're feeding. Your pan will have to fit comfortably and bake evenly in your oven.
You can make several smaller cakes and either serve them individually or stack them.
Re: Other good dessert ideas.
With all the great fresh fruit in season right now, a good alternative might be a fresh fruit crisp or cobbler baked in 2 or 3 11 x 14 oven-safe dishes. Peaches and nectarines are at their peak, and apples are on their way in, too.
If you haven't got a recipe, the old Joy of Cooking has a great basic crust for crisp under (I think) "Quick Cherry Crunch", or just look up cherries. I use the lesser amount of sugar because it's a pretty sweet mix. You'll also find a cobbler recipe in both the old and new books.
Wilton makes a large sheetcake pan for home use. You might have to make a couple of them to feed that many people if they are really hungry for cake. I don't remember what size the pan is but I'm sure I can fit a couple of 9x13 pans in it comfortably.
To echo the other poster, make sure whatever pan you get fits in your oven! Also, you'll be making a lot of batter and icing, so make sure you have the bowls for it too.
Check out "The Cake Bible" by Rose Levy Beranbaum. In the section on wedding cakes she has a discussion of cake pan sizes, how much batter is needed to fill them, and how many people they serve.
If you don't know the capacity of a cake pan you have, you can measure it by filling the pan with water and measuring the amount it holds. Then compare it to a standard size pan, such as an 8 x 1" round pan, to see how much you have to increase the recipe.