Hello,
I had the same problem as you guys and I have managed to find the solution. You have to go to the folder where Construct was installed in your PC. Then, go to > Exporters>html5>plugins>textboox. And you should open the file "runtime.js".
In the script, you should find a place where is says the following:
if(this.autoFontSize)
jQuery(this.elm).css("font-size", ((this.layergetScale(true) / .....
You should edit this text and place the following:
if(this.autoFontSize)
jQuery(this.elm).css("font-size", (((2*this.layergetScale(true)) / .....
The method consists on editing the plugin so that it can increase the size of the text and then it will continue to resize depending on the layout scale.
IMPORTANT: You should close your project and when you open it again, and you press RUN, it will work.