Wow!  Give yourself a gold star for asking such an awesome question.  I, too, vividly remember the days of larger party sizes!  After all, what's a keg of beer if not shared with a dozen or so other friends all packed into your tiny bachelor pad?  Oh, wait, you mean RPG parties!  Yes!  I remember when they regularly featured four to five characters as well.  I can see why you might feel constrained by the dominating shift to three characters at a time in battle. 

If I go far back in my memory, I remember computer RPGs having anywhere from six to eight. Wizardry and the D&D Gold box computer RPGs I played had larger party sizes.  Imagine my reaction when I played Final Fantasy for the first time and I saw that I could only make four characters!  Then, after two consoles (NES and SNES) of having four to five characters in party at a time, Final Fantasy VII dropped me to three.  To be fair, the pump was being primed.  I believe the first SNES RPG I played (Final Fantasy IV) had five, Final Fantasy VI had four and Chrono Trigger had three.

Regardless, by the PSX era, it seemed that just about every JRPG outside of the tactical variety had only three party members.  We can only conjecture why this is so!  However, I believe I have a few strong guesses here.  I believe that, while a few SNES JRPGs started this trend, it was really Final Fantasy VII that cemented the three party member (in battle) standard.  Its commercial success made it that 'one to beat,' so a lot of games tried to copy some (if not all) of its design decisions.

First, when the PSX hit, I believe games like Final Fantasy VII pushed the hardware to its limit at times.  I remember the frame rate dipping on some of those battle scenes.  Perhaps reducing the party members active in battle kept those dips within tolerance.

Second, Square wanted to insure that Final Fantasy VII appealed to a wider audience.  Reducing the active party members in battle meant less to manage and plan on the fly.  And, it worked!  With a stronger focus on art, graphics, and story, Final Fantasy VII had a stronger overall appeal (and sales) than its predecessors.

Third, it made battles faster and shorter.  Again, appealing to a broader audience, long or methodical battle systems had to become streamlined.  Reducing the party member by one more did just that.

This has always been the approach of JRPGs.  In the early Final Fantasy and Dragon Quest games, simplified battle systems (compared to the pen and paper counter parts these games came from) with smaller parties broadened their appeal to the mass market.  Of course, hardware limitation was a consideration, but it wasn't really a limiting factor most of the time.  Games like Pool of Radiance proved that tactically deep RPGs with larger parties could be done, even on the NES.  However, those games never had the mass appeal that simpler games had.  I remember seeing an interview with one of the creators of Final Fantasy or Dragon Quest pointing to this reason (mass appeal), and not technical limitations, as the primary reason for creating a simplified approach to RPG combat.

You can't find them because you don't know where to look.  But do not fear!  While many of my friends know me for not finishing many games, the reality is I have played a metric ton of RPGs, so I have a great idea of what's out there.  While they may be a little harder to locate, you have plenty of options.  Many of the games in the following series have four or more party members in combat at a time: Shin Megami Tensai, Persona, Suikoden, Ar Tonelico, Dragon Quest, as well as many of the Tales of games.  Also, check out Final Fantasy IX and Arc Rise Fantasia.  These are just the ones I remember... if you look hard, you can find many more, I'm sure!

Time, my friend.  That, and the fact that you spoke about it SO much on RPGBacktrack, I felt like I have already played it!  Currently, Fire Emblem games have been doing a great job of filling that portable tactical RPG gaming itch I have from time to time.

Absolutely.  The second game mixes up the mechanics a bit.  Every time you move a character, the computer moves a monster at the same time.  This adds another strategic layer as you try to guess the computer's next move.  The more you play, the better you become at this.  While fans did not welcome this change with universal appeal, I really enjoyed it myself.  If nothing else, it is original.  Vandal Hearts II is one of the few games I have actual beat.

Admittedly, I have never touched this title.

I could not shed tears over it, so I put a dab of water on my cheeks… to make it look like I care.

I can!  If my memory has not failed me, I want to say Drakensang: The Dark Eye and its various sequels/prequels would fit the mold.  The game hails from Germany and is based off a pen and paper system which is popular there.  Expect a deep character creation model typical of pen and paper games, as well as a pseudo real time battle system reminiscent of Baldur's Gate or Knights of the Old Republic.  The story, unfortunately, does not approach the quality of The Witcher, but the gameplay mechanics are quite deep.

I have noticed that myself.  I believe, given the anticipate success of Project X Zone, we need another mash-up RPG!  This one would feature the now divorced parents of various RPG children we never met before.  How awesome would THAT be?

I do not know.  Some crimes are senseless and without explanation.

I did not care for it either.  Maybe that's why we never saw another one.

Let me look into my crystal ball… hmmm… Yes!  I see it!  We will get Knights of the Old Republic 3!  However, this one will be slightly different.  Expect to see it transformed into a third person shooter with cover mechanics.  The creators will claim it is still an RPG though, since you earn experience points, equip items, and occasionally make a choice that may or may not impact the plot.

Dragon Quest games have never sold as well stateside as they do across the pond.  I'm guessing that since Dragon Quest X didn't do so great over there, they believe it would be an utter failure over here.  Pity.

Not unless you play a western RPG such as the acclaimed Wizardry 8, and proceed to name two of your six party members “Ryu” and “Ken,” hahahaha!