I was asked to make the cake for a friend's 80th birthday. It's going to be a large party with about 100 people there. He likes fishing, he's a good carpenter, and he was in the Marines. I don't know if I should do a themed cake or not or how big the cake should be. Any ideas would be greatly appreciated.

If you are doing teirs you could dedicate each one to a specific hobby.
Not sure what you are thinking size wise. I just did a wedding cake for 100 people and used 12", 10", and 8" rounds. Square cakes of course will yield more servings.

You haven't mentioned your experience level. If you can handle tiers, and have experience, that makes a nice presentation. But if you are uncomfortable with that, and have no experience with tiers, I think a sheet cake (or two) might be a better goal. A 2 layer 9x13 will serve 48 (1x2x4). Two of those will serve 96. A 2 layer 12x18 will serve 108 (1x2x4), but it is huge and heavy and awkward to work with.

For an 80th birthday, when a person has lots of interests, I like to do a "life quilt" divided into squares with each square containing a milestone or interest. Here's an example:
http://www.sugarfrosted.com/displayimage.php?pos=-1465

Click on the picture to make it bigger to see the detail.
If you do two 2 layer 9x13 cakes, you could ice them together on one big heavy duty board, as if they are one big cake. You could do different flavors in each cake.

Your board needs to be masonite or wood or even a big tray or serving platter. Cardboard just won't be strong enough, unless you layer a bunch of them glued together.

If you need more help, please ask.

Good Luck! Let us know what you decide to do and don't forget to post pictures!