If you are going to build too small of a wagon you might want to think about downsizing the dimensions of your lumber so it is more correctly sized for the scale of the wagon you are building. Using five 4 x 4's on a very small wagon or 2" flooring would likely look disproportionate to the overall size of the wagon.
I've built a lot of wagons and more is not always better. For a small wagon with a plank floor, I'm thinking 2" is too much. And if you build only 4' wide, I'd rip the 4 x 4's smaller--or at least think about it. Maybe 3 x 4's.
If it were me, I'd scour the net as well as old advertising materials to find a look I liked and then would faithfully duplicate the wagon and it's dimensions. Worth noting here is that some IH sketches I have seen looked to be not fully to scale for the overall scene. Try to find a photo; maybe talk with some diehard IH club people. What you want is out there somewhere.
Those are things I have done in the past and they seemed to work for me.
Wish I could talk you out of the plastic flooring and back to 1" boards or 3/4" plywood. Heck, you really don't even need PT wood but I'm often in the same boat and use it anyway so I understand. The 1" (3/4") PT deck boards sold are all clears or selects with nary even a small knot so they look great. I used them on a 16 ft double axle trailer 15 years ago and they still look great and hold a 5,000 lb tractor with no trouble. Of course, they are never in the rain.
EDIT--This is a guess at the size but I'm going to say the narrowest hay wagon I could envision looking balanced to a typical wagon gear and normal tires--without downsizing the lumber and frame-- would be 6 1/2 ft wide by maybe 10 ft long. Normally I would say 7 ft wide by 12 ft long but I guess you can go smaller. You can fool with the width as long as the length is proportionate and the length looks best in 12, 14 or 16 but I suppose you could get away with 10 ft.
And what most people comment favorable about is a wagon with a flat floor with trim that extends the width of the 4x4 and leaves the stringers exposed on the ends. Then you tuck the lights into the corner of the stringers, so they don't get broken and you also have a nice flat surface for mounting the SMV. I've got pics if you want. Just my two cents, subjective opinion here.
