edit text font size for all text boxes?

You can use JavaScript to reduce every field's font size by looping over all fields in a document, and then if a field is of type text, you reduce the font size:

for (var i=0; i<this.numFields; i++) {
    var f = this.getField(this.getNthFieldName(i));
    if (f.type == "text") {
        f.textSize = f.textSize - 1;
    }
}

You can of course come up with a more sophisticated algorithm for the new font size (e.g. if it's 12, go to 10, if it's 10, go to 9). You can run this in a Batch Process.