I want it to be at least 3.5" in blade length, ideally larger! Does such a thing exist? I've spent a solid couple hours searching through several vendors and have yet to find quite what I'm looking for.
You've been getting some great advice but I thought I'd toss out a bit of a dissenting opinion on the issue of size...
Yes, traditionals are predominantly smaller and it makes sense since they were designed for
pocket carry and, quite often, it could be assumed that a suitable fixed blade was readily available.
IMO, the Buck 110 really paved the way for getting rid of the fixed blade as a common part of working life and after that, modern lockers could show up.
And so here we have an interesting juxtaposition... the modern fixed blade replacement (the large locking folder) and the small pocket knife (often, traditional).
Looking at it this way, it might be easier to see how and why large traditionals are less common. There are 3 common methods of carry. First is via a pocket clip (bzzt) which instantly renders the knife as modern and not traditional. The second is sheath/holster carry. The Buck 110 is really big and heavy enough to almost require sheath carry. And the third is loose, in the pocket.
So this may be something to think about.... Can you adjust to sheath/holster carry? Can you adjust to pocket carry (of a big knife)?
If you don't know, my recommendation would be to spend some money on some inexpensive big knives just to see if you can deal with the change in carry style. The Buck 110 will introduce you to holster carry and the Case (large) Sodbuster will introduce you to large knife pocket carry. Of course, you can try clipless carry with any clipped knife too (my name is Capt. Obvious).
The last point to make here is that those of us who like large traditionals are in the minority and there's just so much more variation in patterns and blade options in the smaller sizes. So, finding exactly what you are asking for may take some more work. Not saying that it's not out there, but just by its size, you're already hunting for something less common.
Not what you are looking for but gratuitous large pocket knife picture...
Untitled by
Pinnah, on Flickr