Change Font Size in Placeholder

I have the following problem. I want to make a Textfield with a placeholder with textSize = 8. But when the user starts typing the cursor it has to grow to a textSize = 10.
I have used the script in format

if (event.value == "")
{
event.target.textSize = 8;
event.value = "Placeholder";
}
else
{
event.target.textSize = 10;
}

I have also used the activate field action and have put the following:
var field1 = this.getField ("Text1");
field1.textSize = 10;
But it does not work. the size does change but when I click outside the textfield. However,  I want it to change when the user starts typing in it

Thanks for the ideas