There are so many parameters to consider in determining the "best" monitor size, it could take a dozen hours of research to sort through all the variables. And, even then, personal opinion would come into play. If possible, I would avoid CRT completely if you have an LCD display with a digital connection. However, if you are planning on using an older Macintosh, this probably isn't an option since standard digital connections like DVI didn't arrive until late last century.

That being said, CRT technology improved over time. So, regardless of the size, I would try to use a newer CRT display. If you are hoping for a more authentic experience, consider this: except for the earliest models and 6400/6500 models, most Performa computers had either a 14" or 15" display, built-in or bundled. So, if authenticity is your goal, I would go with a 15" display. If quality is your goal, go with the newest 15" or 17" display you can find. Newer displays are likely to have higher refresh rates, lower dot pitch, lower grille pitch... I could go on. However, I'm sure you can find someone out there that will have different thoughts on the matter.

Also keep in mind that some Apple monitors, and probably some non-Apple monitors, may have system requirements for their drivers. For example, the AppleVision 750 required at least a 68040 CPU and System 7.1, and the Multiple Scan 720 required at least a 68040 CPU and System 7.5. However, this may not matter in your case, and certainly would only matter if you were using an actual Macintosh computer.

What machine are you on? Old Macintosh video output is "sync on green" which was a problem with some CRTs.
I don't think you will observe toooo much a difference @ 640x480 8bit graphics. The larger the display, the uglier the pictures, I suppose.
If you don't want (like) a TFT, my first choice were Sonys with their rectangular dot matrixes those days. Today I am using a small FSI TFT with an adapter at my PPC7100.
But like InkBlot said, it is always a matter of taste.

Those Apple Sony monitors were pretty awesome. I have one that's something like 13" (640x480) and it still looks great. I swear Sony built those Trinitrons to last forever. My grandmother still has a Trinitron TV from the late '70s!!

Yep, for DB-15 connections, the old trinitron RGB monitors.

for ADC or VGA, the Apple 17" Studio Display. Big, bright, sharp, fast, and cheap. I sold 2 for £0.99 a few months ago cos I needed the space.

The first generation Apple Studio Display is LCD and has a DA-15 (the standard video port on all Macs until the iMac). It is a 1024x768 monitor. They are occasionally available on eBay. Look for its model number (M4551 Rev. A). Rev. A is important because it's the only one with a DA-15 port. The rest come with VGA or ADC ports

I drive 2 Philips 170 S4 (17" LCDs) from my IIfx. I have a Supermac Thunder/24 and a Thunder/8 Nubus card. The former is not at all bad at 1024x768 and millions of colours, the latter good at 640x480 and 256 colours. I have driven one of these from an LCIII and it was most respectable.

My guess is that any of the old production-level CRTs will have sharper picture and better colours although a lot depends upon your video card. These old monitors are *massive* though. Even the 13" monitors are big and they consume 10-times the power of a flat-screen.

If you look on eBay and the like, you may come across people willing to give them away, so long as they find a good home.

I may be mistaken, but your Performa 6116 seems to have 640k video memory from factory.
You might better stick to small displays, because the maximum pixels and/or colors will be very limited.
The primary (DRAM) video output will drive resolutions up to 832x624, so if you don't own an AV or HPV card with more VRAM on it, try to stay happy just as it is.

Those Apple Sony monitors were pretty awesome. I have one that's something like 13" (640x480) and it still looks great. I swear Sony built those Trinitrons to last forever. My grandmother still has a Trinitron TV from the late '70s!!

I opened my Apple 13" RGB Trinitron monitor once to clean it out, but the thing was built like a tank inside. It had three or four separate circuit boards on various sides of the case, each separately encased in a strong metal cage with holes in it. Still works decently even though it's about 19 years old.

Im was using a Sony Multiscan 200ES 17" with my G3 266 with a Viewsonic VGA adapter, and never get problems (changing scales of display resolution or colors).
In the early 90s, the apple display 15 (the one from my old LC lll) and above, they were all Trinitrons from sony.
Now im using the sony multiscan monitor with my quicksilver and no complains at all.
it keeps an absolute colorimetric exactitude (if you do graphics or smthing similar)
It worths a try, its a really cheap and high quality CRT.