Going throught the
EXACT same thing. I love my car so it really is very upsetting to me. Anyhow...
Talk to people you know who have bigger cars. Get their take on them, pros and cons. Ask the big question, "if you had it to do over again, would you buy the exact same make and model?"
In order to save yourself some time you may want to actually do this step first,
pick up a Consumer Reports guide on cars!! Mine is on SUVs, Wagons, Minivans, & Trucks exclusively. Consumer Reports also has a website, but I'm not sure what info is available for free & what you have to pay for. Besides, I find the book (magazine) easier, it's much more portable than my computer.

I read mine cover to cover only skipping those vehicles that I knew wouldn't fit the car seats & those of us not in car seats.
Next, go to the dealerships (devote a day depending on how many models make your list), looking at those models. Many you'll take a look at and decide to forget it, others you'll test drive. I had 8 on my list--3 vans & 5 SUVs. After this day we had it down to one van (Honda Odyssey) and one SUV (Toyota Sequioa).
Then, go to the websites. Build & price the cars in the running. Figure out the values (MSRP, Dealer Invoice, Wholesale [this includes holdbacks & dealer incentives]) on your new car and the value of your old car. Edmunds.com, kelly Blue Book, and the National Automobile Dealers Association are great on this. I highly recommend the new car reports from consumer reports for the new car, but skip it for the trade in. It is amazing that by knowing the numbers what a much better deal I was getting. Dealerships dropped down prices by thousands after they figured out that I knew.
Decide what your first choice is (mine is the Sequioa, but do to the fact the van is cheaper to gas up, insure, maintain, & purchase in the first place & probably more convienent with very little kids, I'm getting the van) and go get quotes from dealers. My highest quotes came from the internet sources of carsdirect.com and car.com, FYI. My best quotes came from using the "find a dealership" & "get a quote" button on the official manufacturer website for the vehicle. And that was easier since I did not have to go in and go through all the muck that goes with going into a dealership & could/would shop dealerships further out. I don't mind driving an hour to save $2000, but not just to go bargain in the first place. Save this for when the decision has been made. My middle quotes came from the dealerships I walked in to. My quotes varied dramatically. From full MSRP, and above a couple of times, to just about wholesale.
Now is a great time to buy a 2005 new. Either dealerships need to get rid of it because the 2006 is out or is about to be. That is a bargaining tool in and of itself. Sales are also very lull.
Most importantly, take your time and don't be pushed. And when you make your decision, stick to your guns! I had one dealership that gave me a great price, but then wouldn't dealertrade to get me the interior color I wanted after they said they would. I walked out, and amazingly enough, got a better deal (by only $500), but hey, it was wholesale price exactly--they needed to just reduced the 2005 inventory.
Sorry so long, but that is the info I have for you.