The drop-down box embeds instructions in the text to change the font size when people read the post... and not the edit box. I'm going to hazard a guess that the problem here is that the default edit box text is too small.
If you're on Chrome or Firefox then there's some "make all the text bigger/smaller" keys. On UNIX it's control and plus/minus. On the Mac it's command plus/minus. On Windows it might be either control or windows and plus/minus. That's definitely only a rendering change. There's also a menu option (under view) for this.
There's also, in both browsers, an option to set a minimum text size. If you set this to (say) 14 or 16 or even larger, when a website says "do this in really really small", your web-browser will just say "erm. No." and do it in normal size text.
On my version of Firefox it's under Edit/Preferences, then pick Content at the top, then look for Fonts and click Advanced... and there's a "Minimum font size" selector.
For Chrome it's spanner/settings/show advanced settings and then "web content" and push "customize fonts". Scroll down in that window and; minimum font size.