MAILTO max-length of each internet browsers?

Safari and most email clients have no hard limit (depends on available CPU and RAM)

2015 Web Browser Testing:

• Safari
  • 705000000
  • 2 minutes
  • Mozilla/5.0 (Macintosh; Intel Mac OS X 10_10_5) AppleWebKit/601.1.56 (KHTML, like Gecko) Version/9.0 Safari/601.1.56
  • limited by 16GB RAM
• Firefox
  • 268435455
  • 20 seconds
  • Mozilla/5.0 (Macintosh; Intel Mac OS X 10.10; rv:41.0) Gecko/20100101 Firefox/41.0
  • limited by maximum string length
• Chrome
  • 2097132
  • 1 second
  • Mozilla/5.0 (Macintosh; Intel Mac OS X 10_10_5) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/45.0.2454.101 Safari/537.36
  • limited without explanation
• IE
  • 2029
  • 5 seconds
  • Mozilla/5.0 (Windows NT 10.0; WOW64; Trident/7.0; .NET4.0C; .NET4.0E; rv:11.0) like Gecko
  • limited without explanation

2015 Email Client Testing:

• Mozilla Thunderbird
  • 2097132 works in 1 second
  • 268435455 uses 100% CPU for 2 minutes but fails to render the body and is not usable
  • version 38.3.0
• SeaMonkey
  • 2097132 works in 5 seconds
  • 268435455 uses 100% CPU for a long time (more than 5 minutes)
  • version 2.38
• Apple Mail
  • 500000 works in 14 seconds
  • 2097132 uses 100% CPU for a long time (more than 5 minutes)
  • version 8.2
• Microsoft Outlook
  • trims to 2070 in 1 second
  • version 2013

2017 update

Chrome 61 still works with 2097132 length on macOS (16 GB RAM);

navigator.userAgent;"Mozilla/5.0 (Macintosh; Intel Mac OS X 10_13_0) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/61.0.3163.100 Safari/537.36"location.href='mailto:?body='+'a'.repeat(2097132);

According to @Chloe on windows (32 GB RAM) it's limited to 1800;

"Mozilla/5.0 (Windows NT 6.3; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/61.0.3163.100 Safari/537.36"

Just for reference I've found that google chrome's latest build (23.0.1271.95) only lets me generate a mailto with a maximum length of:

1910 characters

Firefox does not impose any limits on the length of URLs other than the system's available memory, so you'll have to look into operating system limits for the maximum length of the parameters when invoking a process.

Additionally, you can set a Web app as your mailto URI scheme handler, and in that case the limit depends on the server accepting the request. Firefox comes with a few options like Gmail and Yahoo! Mail, and you can register your own handler with navigator.registerProtocolHandler. If you're curious, open about:config and filter on "gecko.handlerService.schemes" to see the Web app handlers you have configured in your browser.

I can't say for other browsers, but once you start to depend on so many different variables (browser, available memory, OS, server), it's probably best to try a different strategy.