I think you may want to do a bit more research before diving in.
Also, do not take anyone's words as completely true or good advice.
Im going tofirst refute the comments said already:
1) Controls are the most important feature of your machine. You need to know which games you want to play, and what are the best controllers to use for them. Then you need to make a comfortable layout with them.
2) 19" monitors are Not too small. Pretty much every arcade game from the 80s, and many well into the 90s... used 19" arcade monitors. It was pretty much the standard monitor size for the longest time.
To know what size monitor... you need to know what kind of cabinet you are going to build.
A) If its based on an older classic cabinet such as Galaga... then 19" is the perfect size. These cabinets have you very close to the monitor.. and only 1 person is playing on them at one time. Galaga has a cabinet that has enclosed side-walls. Your head is pretty much stuck inside.. and it helps to block out distractions of other people, other sounds...etc.. and keeps you far more focused on the game you are playing.
The sides also:
1) Block glare from room lighting /sunlight
2) Makes a nice reflective channel for sound to bounce around in.. directing it right to your ears, rather than leaking in other directions.
3) Can sort of amplify the sounds, due to the shape of the cabinet. Further can create resonate effects, from both the inner part of the cabinet, as well as the sidewall area.
B) Some classics like Robotron have dual player setups, and do not have the side-walls. ... But again, they do have the monitor very close to your eyes... so 19" again works perfectly. Making the monitor too large, when its too close to your eyes.. would make you have to pan around to see everything. That would result in a lot of gameplay problems... leading you to early deaths.
C) Games like Mortal Kombat often used 25" monitors. They were further away from your eyes... and the cabinet did not have enclosed sides... allowing for many people to see the screen from nearly every angle.
D) Some Fighting games and other later era machines used Pedestal style cabinets. They have larger monitors, placed many feet away from the player. This was used a lot of the time to attract large groups of people, so that everyone could see the gameplay. It was also used in some simulator games, to give a larger sense of motion.
E) The end of the Arcade era, have used High Resolution monitors.. such as LCDs. This is often seen on more modern racing games.
These work good for being able to run PC games on your machine, as well as older emulated games. However... the look they present may not be completely favorable on the older games. Especially because LCDs do not look very good when not being used at their naitive resolution. Even then... its a completely different display technology, so its always going to make the games look too 'Clean', 'blocky', and the colors will not be correct.
F) Then you have to decide if you want to see about using a real acracde monitor, and old CRT TV, or an LCD. Each option has positive options, as well as negatives & drawbacks.
Other disputes:
Is the space too big - This is Irrelevant to functionality. Games were only optimized for space, so that the Operators had more room for more games. Many of the games were build with young and thin kids in mind as well. Theres no reason why you should limit comfort, just because the various people here do not care for the look or the relative size compared to the Arcade machine sizes.
Looks take Priority - I also disagree with this. Function is far more important than looks. You can have a pretty looking car... but if its missing brakes, has horribly uncomfortable seats that do not adjust far enough back (cramped), and handles like a freight truck... then its not going to be enjoyable, no matter how nice it looks.
I think the best thing to do... is start playing some of the different games that use alternate controls, such as spinner and trackball games.
See if you like them... because you may quickly find out that you really want to have the proper controllers to use them on your machine.
Then look into games that used combination of unique controllers... such as Tron, which has a Spinner and a Trigger stick. Actually, Mad Planets has the same controller setup.. and its awesome fun.
Do you like shooters like Galaga? If so, you might want at least one classic leaf-switch button to play it with.. instead of the typical microswitches. Rapidfire games can cause fatigue to your hands quickly.. if you use microswitches. Leafs are much lighter to depress.. and you can lightly bounce or "Vibrate" them on and off with almost no effort at all. However, leaf controls such as joysticks, are not always as accurate... so they are not really optimal on direction critical games like Fighters.
Do you like Ms Pacman? Games like this used a 4 way joystick. It makes turning the corners in the game quicker and easier. While some joysitcks can be switched between 4 and 8 way modes... its best to have one that is Physically Restricted.
Do you like Analog games? Games like Outrun used a 270 degree steering wheel and analog pedals. Afterburner II used a flight stick. Some of these games will not play well using digital controllers.
And there are also special driving games that use a 360 degree wheel. (If can rotate in either direction, infinitely, without any restriction) Games like SuperSprint used them, and are a blast to play. You can use spinners to play these games as well. A full wheel version might be preferable, depending on your tastes.
There are controls like shifters to consider.
And there are unique controllers that are specific to one particular game. For example... 720 degrees used a very unique spinner that has an angled handle attached to it. Sinistar used a 49 way stick.. that has a special rubber spring system... for a LOT more control of the ship. There are Pull and Release controllers for golf, baseball, and simulated pinball games.
And far more, than Id ever have time to type out.
Look for the links to special controller based games, and start playing them. Then you have some form of base to form what you want to build... and can get better help in that direction.
If you just want something quick... you can always just make a stand-alone control panel too. It could be a simple prototype for testing / playing... until you finalize your opinions and tests.