The maximum font-size for emoji in Mobile Chrome 69
If you set it to 147px, the emoji are not rendered properly. Here's the bug report, and here's a Stack Overflow page.
At font-size 146px, the emoji seem a bit blurry on that setup. Best to test.
Mobile Safari 11 also supports the 146px.
Mobile Opera 47
garbles large emoji.
At font-size 146px it doesn't display them at all.
Here's the bug report (also reported at bugs.opera.com but the reports aren't public there).
The maximum font-size for emoji in Mobile Firefox 62
If you set it to 99px, the emoji are not shown. Here's the bug report.
Also test in the latest version of the "Samsung Internet" browser.