How to change the hint size of TextInputLayout

This is my xml

<android.support.design.widget.TextInputLayout
                style="@style/main_input_text"
                android:layout_marginTop="@dimen/activity_medium_margin"
                app:hintTextAppearance="@style/TextAppearence.App.TextInputLayout">

                <android.support.v7.widget.AppCompatEditText
                    style="@style/main_edit_text"
                    android:inputType="text" />

</android.support.design.widget.TextInputLayout>

And this is style

<style name="main_input_text">
        <item name="android:layout_width">match_parent</item>
        <item name="android:layout_height">@dimen/activity_edittext_height</item>
        <item name="android:background">@drawable/mc_shape_border_button_transparent</item>
        <item name="android:gravity">center_vertical</item>
        <item name="android:paddingTop">@dimen/activity_extra_small_margin</item>
        <item name="android:paddingBottom">@dimen/activity_extra_small_margin</item>
        <item name="android:keyTextSize">8sp</item>
        <item name="android:textSize">8sp</item>
</style>

I don't find something like hintSize, textSize and keyTextSize not helping

In your styles.xml

<style name="TextLabel" parent="TextAppearance.Design.Hint">
    <item name="android:textSize">16sp</item>
</style>

And then in your layout file:

<android.support.design.widget.TextInputLayout
         android:layout_width="match_parent"
         android:layout_height="wrap_content"
         android:layout_marginLeft="@dimen/activity_horizontal_margin"
         android:layout_marginRight="@dimen/activity_horizontal_margin"
         android:layout_marginTop="12dp"
         app:hintTextAppearance="@style/TextLabel"
         android:minHeight="30dp">

Change the TextInputLayout's app:errorTextAppearance in XML.

<android.support.design.widget.TextInputLayout
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            app:errorTextAppearance="@style/CustomTextInputLayoutStyle.ErrorTextStyle">

Where CustomTextInputLayoutStyle.ErrorTextStyle is defined in styles.xml as

<style name="CustomTextInputLayoutStyle.ErrorTextStyle" parent="TextAppearance.Design.Error">
        <item name="android:textSize">10sp</item>
</style>

@Santiago Martínez answer only works for hints on non empty Edittext (inside TextInputLayout)

But for empty Edittext, TextInputLayout hint takes property from Edittext textSize.

1. Add hintTextAppearance to your InputLayout with a style

   http://schemas.android.com/apk/res-auto" android:layout_width="match_parent" android:layout_height="wrap_content" app:hintTextAppearance="@style/TextInputLayoutHintText" > android:layout_height="wrap_content" android:textSize="16sp" />

2. Then just add the style to your styles.xml

For material 1.4.0

implementation 'com.google.android.material:material:1.4.0'

You only need to change the android:textSize attribute for the TextInputEditText for the hint size to change.

<com.google.android.material.textfield.TextInputLayout
        style="@style/Widget.MaterialComponents.TextInputLayout.OutlinedBox"
        android:layout_width="match_parent"
        android:layout_height="wrap_content">

        <com.google.android.material.textfield.TextInputEditText
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:textSize="18sp"/>

    </com.google.android.material.textfield.TextInputLayout>

You can reduce the font size on the EditText - that will reduce the size of the hint as well. i.e. android:textSize="16sp"