Adjust icon size of Floating action button (fab)

Question

As your content is 24dp x 24dp you should use 24dp icon. And then set android:scaleType="center" in your ImageButton to avoid auto resize.

Try to use app:maxImageSize="56dp" instead of the above answers after you update your support library to v28.0.0

Make this entry in dimens

<!--Floating action button-->
<dimen name="design_fab_image_size" tools:override="true">36dp</dimen>

Here 36dp is icon size on floating point button. This will set 36dp size for all icons for floating action button.

Updates As Per Comments

If you want to set icon size to particular Floating Action Button just go with Floating action button attributes like app:fabSize="normal" and android:scaleType="center".

  <!--app:fabSize decides size of floating action button You can use normal, auto or mini as per need-->
  app:fabSize="normal" 

  <!--android:scaleType decides how the icon drawable will be scaled on Floating action button. You can use center(to show scr image as original), fitXY, centerCrop, fitCenter, fitEnd, fitStart, centerInside.-->
  android:scaleType="center"

There are three key XML attributes for custom FABs:

• app:fabSize: Either "mini" (40dp), "normal"(56dp)(default) or "auto"
• app:fabCustomSize: This will decide the overall FAB size.
• app:maxImageSize: This will decide the icon size.

---

Example:

app:fabCustomSize="64dp" 
app:maxImageSize="32dp"

The FAB padding (the space between the icon and the background circle, aka ripple) is calculated implicitly by:

4-edge padding = (fabCustomSize - maxImageSize) / 2.0 = 16

---

Note that the margins of the fab can be set by the usual android:margin xml tag properties.

The design guidelines defines two sizes and unless there is a strong reason to deviate from using either, the size can be controlled with the fabSize XML attribute of the FloatingActionButton component.

Consider specifying using either app:fabSize="normal" or app:fabSize="mini", e.g.:

<android.support.design.widget.FloatingActionButton
   android:layout_width="wrap_content"
   android:layout_height="wrap_content"
   android:src="@drawable/ic_done_white_24px"
   app:fabSize="mini" />

<ImageButton
        android:background="@drawable/action_button_bg"
        android:layout_width="56dp"
        android:layout_height="56dp"
        android:padding="16dp"
        android:src="@drawable/ic_add_black_48dp"
        android:scaleType="fitXY"
        android:elevation="8dp"/>

With the background you provided it results in below button on my device (Nexus 7 2012)

Looks good to me.

What's your goal?

If set icon image size to bigger one:

1. Make sure to have a bigger image size than your target size (so you can set max image size for your icon)

2. My target icon image size is 84dp & fab size is 112dp:

   <android.support.design.widget.FloatingActionButton
       android:layout_width="wrap_content"
       android:layout_height="wrap_content"
       android:layout_gravity="center"
       android:src= <image here>
       app:fabCustomSize="112dp"
       app:fabSize="auto"
       app:maxImageSize="84dp" />