Thanks for the replies!
I'm pretty stuck on 4 players though. Even if it only gets used a fraction of the time, being able to hop on an play 3 or 4 players whenever the opportunity arises makes it worth it to me. (And building separate CP's for P3 and P4 seems like it would be even more work than just building the one?)
So, what I've gathered so far ;
40" wide CP should be long enough.
Add a 4th button for 3 and 4.
Angled controls are debatable.
Yah?
Oh, I 'found' KLOV and it lets me search by number of players, so I've gathered the answer to my own question there! I'll post a list to save anyone else the trouble.

I'll sort by years since I can only get 50 results at a time (There were a lot of 4 player games made! Wow)
4 Player Games and the buttons they use sorted by years :
19XX-1980 (We'll call this the 'Spinner' Years. Jesus there were a lot of "Tennis" games before 1980....or at least games that used nothing but a Spinner.
Atari Football - 1 ? (Hard to tell from the pictures. Uses 4 trackballs. COCKTAIL)
Atari Soccer - 1 ? (Hard to tell from the pictures. Uses 4 trackballs. COCKTAIL)
Barricade - 4 (U/D/L/R May be able to use a joystick for this?)
BiPlane 4 - 1 (2 Joysticks per player)
Car Polo - 0 (1 Pedal and 1 Wheel per player. COCKTAIL)
Challenge - 0 (1 Spinner per player. COCKTAIL)
Checkmate - 0
Ckidzo - 0
Clean Sweep - 0 (Hard to tell from the pictures. Appears to use a spinner per player)
Comotion - 4 (U/D/L/R May be able to use a joystick for this? COCKTAIL)
Cops 'N Robbers - 1
Countdown - 0 (1 Spinner per player? No picture.)
Elimination - 0 (1 Spinner per player. COCKTAIL)
Fisco 400 - ?? (Uses steering wheels)
Fun Four - 0 (1 Spinner per player? No picture.)
Goal IV - 1 (1 Spinner per player? COCKTAIL)
Hesitation - 0 (Hard to tell from the pictures. Appears to use a spinner per player)
Hit Me - ?? (No picture. No clue.)
Indy 4 - 0 (1 Pedal and 1 Wheel per player. COCKTAIL)
Knockout - 0 (1 Spinner per player? No picture. )
Leader - 0 (1 Spinner per player)
Meadows 4 in 1 - 0 (1 Analog per player. COCKTAIL)
Olympic Tennis - 0 (1 Spinner per player)
Pace Car Pro - 0
Pong Doubles - 0 (1 Spinner per player. COCKTAIL)
Quadrapong - 0 (1 Spinner per player. COCKTAIL)
Scoring - ??
Sea Battle - 1
Shark - 4 (U/D/L/R May be able to use a joystick for this? COCKTAIL)
Soccer - 0 (1 Spinner per player.)
Sprint 4 - 0 (1 Pedal, 1 Wheel and 1 4 way shifter per player)
Survival - 0 (1 analog per player. COCKTAIL)
T.V. Tennis - 0 (1 analog per player)
Table Tennis - 0 (1 Spinner per player. COCKTAIL)
Take 7 - 0 (1 Spinner per player.)
Tennis Tourney - 0 (1 Spinner per player)
Tournament Table - 1 (1 Spinner per player)
TV Joker - 0 (1 Spinner per player)
Video Action - 0 (1 Spinner per player. COCKTAIL)
Video Action 2 - ?? (1 Spinner per player. COCKTAIL)
Warlords - 1 (1 Spinner per player. COCKTAIL)
Wimbledon - 0 (1 Spinner per player.)
Winner 4 - ?? (2 Spinners per player?)
Wipe out - 0 (1 Spinner per player)
1981-1985Gauntlet - 2
Vs. Mahjong - 2 (2 Screens ?)
Vs. Tennis - 2 ( 2 Screens ?)
1986-1990Cadash - 2 (2 Screens)
Center Court - 0 (8 way and 4 way per player. COCKTAIL)
Crime Fighters - 2
Cyberball - 1 (2 Screens)
Cyberball 2072 - 1
DD Crew - 2?
Dark Seal - 2
Dunk Shot - 2 (Trackball per player. COCKTAIL)
Euro Football Champ - 2
Face Off - 1
Football Champ - 2
Gauntlet 2 - 2
Gold Medalist - 3?
Growl - 2
Hattrick Hero - 2
High Impact Football - 1
Hit the Ice - 2
Hot Rod - 0 (1 Pedal, 1 Steering Wheel per player. COCKTAIL)
Hyper Sports Special - 3
John Elway's Team Quarterback - 1
The Main Event - 1
Mexico 86 - ?
The Ninja Kids - 2
Punk Shot - 3
Quartet - 1
Ring No Ouja - 2
Runark - 2
Tecmo Bowl - 2
Tecmo Bowl II - 2?
Teenage Mutant (Hero) Ninja Turtles - 2
Touchdown Fever - 2
Touchdown Fever II - 2
Tournament Cyberball 2072 - 1 (2 Screens)
Trog - 1
V'Ball - 2
World Soccer Finals - 2
1991-1992Arabian Fight - 2
Arabian Magic - 3
Atomic Punk - 1?
Atomic Punk Global Quest - 2
Bomber Man World - 2
Bucky O'Hare - 3
Captain America and the Avengers - 2
Captain Commando - 2
Crime Fighters 2 - 2
Dead Connection - 2
Desert Assault - 2
Dynablaster - 2
Euro Champ 92 - 2
Exbania - 2
Exvania - 2
G.I. Joe - 2
Golden Axe : The Revenge of Death Adder - 3
Heated Barrel - ?
Hook - 2
Karate Blazers - ?
Rim Rockin' Basketball - 2
Ring Rage - 2
Roller Games - 2
Silent Dragon - 3
Spiderman - 2
Sunset Riders - 3
Super High Impact - 1
Teenage Mutant (Hero) Ninja Turtles : Turtles in Time - 2
The Simpsons - 2
Vendetta - 2
Wild West C.O.W.Boys of Moo Mesa - 2
WWF Wrestlefest - 2
1993-19952 on 2 Open Ice Challenge - 3
Blazing Tornado - 3
CB System II - 6!!!!
Dungeons and Dragons : Tower of Doom - 4 (One good reason to have 4!)
Dunkmania - 3
Hattrick Hero '95 - 3
Knuckleheads - 3
Metamorphic Force - 2
Muscle Bomber - The Body Explosion - 3
Muscle Bomber Duo - Heat Up Warriors - 3
Muscle Bomber Duo - Ultimate Team Battle - 3
Mystic Warriors - 2
NBA Jam - 3
NBA Jam Tournament Edition - 3
Run & Gun - 3 (4 Player seems to require 2 screens)
Saturday Night Slam Masters - 3
Snow Bros. 2- With New Elves - ?
Super Cup Finals - 3
World Cup 94' - 2
Zero Team - 2
1996-20003 on 3 Dunk Madness - ?
Battle Circuit - 2
Dungeons and Dragons : Shadow of Mystara - 4 (2 Screens?)
Gauntlet Dark Legacy - 3
Gauntlet Legends - 3
Hyper Athlete - 3
NBA Hangtime - 3
NBA Jam Extreme - 4
NBA MAximum Hangtime - 3
NBA Showtime - NBA on NBC - 3
NFL Blitz '99 - 3
NFL Blitz 2000 - 3
NFL Blitz 2000 : Gold Edition - 3
Oriental Legend - ?
Run & Gun 2 - 3
Rushing Heroes - ?
The Age of Heroes - ?
Wayne Gretzky's 3D Hockey - 3
WWF Royal Rumble - 3
2001 and beyond!?!?
I'll have to finish this later. I gotta run for now. ITS QUITTIN TIME!