Welcome to the Cult of Teardrop!
I'm building a 5 footer over a HF 4 foot wide trailer. That put my side walls right about the center line of the tire. You can see how I am dealing with it.
The "fender" is built up of 4/4 mahogany, covered with 4/4 purpleheart and maple plugs for the screw holes.
As to why people build a second, third and so forth. Experience is the best teacher. Something like this has to be done at least 3 times: 1 to make mistakes, 2 to learn from those mistakes, and 3 to make fewer mistakes. That said, we decided our tear would be something we could be proud of and are using the best materials and techniques we can afford within reason. That doesn't mean we don't make compromises, such as the HF frame. I have another reason for wanting to do this right the first time: I may not get another chance to do something this ambitious. That's another story, though.
Watch the forums for months before you dive in. You can learn a lot. Here are a few things I didn't learn from the forums but am learning now:
1 - it takes a long time. I'm past the 6 month mark and taking time off work to finish this thing by summer.
2 - I would not build out over the wheels for #2. It adds complexity and failure modes you don't have if you build between the wheels. Just before I put the floor on I started worrying about the walls sagging in the wheel area and added all kinds of stuff to strengthen that area. Probably not necessary, but going back later to fix it is impossible.
3 - Try to get 4x10 or 5x10 plywood if you need it. I wish I had tried harder. Seams show no matter how hard you work on them.
4 - Woodies are cool and take a lot longer to build. Skin with aluminum if your time is limited. I thought I would save $. Nope. Time is money, and aluminum can be cool, too.
5 - Don't try to do bent laminations using the trailer as a form. See my album; build a form. The problem is you can't get to both sides to clamp when you bend on the trailer. The inside strips will try to separate as they soak up moisture from the glue and there isn't a thing you can do about it short of putting a screw every couple inches.
6 - Build the hatch completely in place. I built mine half on, half off the trailer. One side sprung out about 3/4 inch. Ouch. It's not fatal -- a good spring hasp will pull it in. But it bites, nonetheless.
7 - The forums are good, but if it doesn't pass the common sense test it probably isn't right. Not everyone who contributes is an expert; some are just opinionated.
8 - Buy all your hardware before cutting the first board. Hardware will dictate things like window placement, wall thickness, wiring locations...
9 - Wait to finish #1 before starting #2

You will want to start #2.
In anything at all, perfection is finally attained not when there is no longer anything to add, but when there is no longer anything to take away.
-- Antoine de Saint-Exupery