Okay, it's not crazy, I totally get it and all those haters out there need to simmer down! ;) I have huge parties too...partitally because our family is HUGE and partially b/c I can tend to go a little crazy. First off remember that everyone you invite won't be able to make it, even if they say they will. You will have at least 10 of those people bail out, I assure you. Our house isn't huge and we managed to do it (the last guest count was about 50), although we did have the backyard to utilize and you won't have that on Dec. 4th. Here's my advice. Clean out your garage. Sweep the floor and if possible get a tarp or some cheap tableclothes (plastic) and hang them over stuff that you don't want kids getting into. Move into the garage some balls, bikes, trikes, etc. If you have some of those little tykes slides and stuff those are great. If you have a rug you can put in there, do but otherwise I wouldn't worry. Parents will be happy about it, don't worry. Mostly they want their kid to go play and you will always have at least one adult in there with the kids to keep an eye on them, it always works out. Spread out toys in the other rooms as well. Since it's winter, you'll probably want to include the birthday boy's rooms in the area of play, but I would certainly close off your room and others you don't want the kids in.
Secondly, get as much of it catered as possible. If you're doing a meal, which you probably are, then check out Costco. It's the cheapest I've found for sandwich trays, etc. Plus they make absolutely AWESOME cake! Set up a buffet table with throw-away or reusable. I've used both, but I prefer the reusable picnic ware. For my daughter's first b-day I made all the food for about 60 people and it was a nightmare. When I did her 2nd b-day catered it was a huge life saver...of course I was 9 months pregnant, but still!
Do as much of it beforehand as possible. About a week before I started cleaning and organizing. I put out all the serving dishes and utensils and labled with small post-its what food would go where. I put together all the party favors about a week to two weeks ahead. Use plastic silverware and get juice boxes for the kids. For the adults I offered beer, water and iced tea. The only cups that were out were for adults.
I am seriously a pro at this...if you want more help, please PM me, I'm happy to offer tips. Yes, this party is for you, but who cares. It's fun and it makes you happy so do it! Plus, it will be really fun for your son to look back on the pics and videos from this day! Do what you want and don't worry about it. When they get older you will have less control over their themes and guest lists, so go crazy while you can...that's my motto at least!
Oh and trust me...all those cousins and aunts and uncles ARE excited about the birthday! If you have a close family, which it sounds like you do, they are excited and they WANT to be there....and if they aren't, they won't come! Everyone looks forward to my big parties, it's part of the price you pay with a big family, but I think it's worth it.