40", 42" or 43"

I'm just going to presume that the size you are indicating is the circumference ...

What is the application of the wheel going to be for?

OR, are you describing the diameter?

Hello!
sorry. Im looking for a wheel that is 40, 42, or 43 inches. Its for my masters dissertation. Im building a new kind of bicycle, a very interesting one. Maybe you will see it on the market in near future. 😉

Do you have any idea?
Best regards
Erik

The biggest wheels I've seen in anything resembling serial production are 36". There are actually at least two sources for those:
dirtysixer.comdirtysixer.comOpen ↗
cokercycles.comcoker monster cruiserOpen ↗

Then there are these guys:
hiwheel.comsuperior.htmOpen ↗
But they don't do pneumatic tires.
Their stock offerings are 38" and 48". But they do offer custom sizes at additional cost.

Extending a rim is probably fairly doable.
And I wouldn't faint at the thought of getting it decently round either.
Hubs, well, it might be possible to get a wheel sufficiently laterally stiff using Fatbike hubs.
Otherwise, widening a hub isn't much of a challenge for anyone with access to a lathe and some round stock.
Spokes would have me quite intimidated though.
Buying a good enough wire and a spoke threader is just money.
But I can't see an easy, low-cost, low volume way of creating the head that normally goes on the hub end of the spoke.
And I don't think you'd be able to convince a spoke manufacturer to sell you some at excess length.
The three options that seems the least hassle is either to make some hubs from scratch, that'll take a straight pull spoke that's threaded at both ends. Or to make some extension sleeves. A normal J-bend spoke at the hub, then instead of a nipple, an extension sleeve. Then another spoke, threaded at both ends that hook on to that. Continue until it reaches the rim.
Or put a deeper, 180 deg bend at the end of the spoke and simply hook it into the hub flange.
Maybe it'll be good enough.
After all, using a spoke with a "staircase" bend was once used as a roadside fix if you broke a driveside spoke.

But beware that inventing something actually new when it comes to ge overall configuration and the mechanics of a bicycle is quite difficult. They've been around for quite some time. Most stuff has been tried(and usually rejected) before.