how to set floating label text size?

The accepted answer helped me a lot to find my solution (i.e. setting the exact font size, not only using small, medium, ...).

In addition, by defining a style for app:hintTextAppearance, you can also simply set the color of floating label :)

in styles.xml:

<style name="CustomTextAppearance" parent="@android:style/TextAppearance">
    <item name="android:textSize">16sp</item>
    <item name="android:textColor">@color/colorAccent</item>
</style>

and in your TextInputLayout:

<android.support.design.widget.TextInputLayout
android:id="@+id/lyt_goal"
style="@style/CustomTextInput"
app:hintTextAppearance="@style/CustomTextAppearance">

<android.support.v7.widget.AppCompatEditText
    android:id="@+id/edt_goal"
    android:hint="@string/str_goal" />

Try following code. This may help you:

<android.support.design.widget.TextInputLayout
        android:id="@+id/input_layout"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        app:hintTextAppearance="@style/TextAppearance.AppCompat.Medium.Inverse">

You may use app:hintTextAppearance="@style/TextAppearance.AppCompat.Small.Inverse" instead.