How to define size of a button in Excel 2013 using customUI editor

Question

I am trying to create a new ribbon with 2 buttons in Excel 2013. I was able to create it using Custom UI editor thanks to Excel CustomUI ribbon layout and How to add a custom Ribbon tab using VBA?

When I input the code as

<button id="aa" label="CORE"  onAction = "HMA_CORE"/> 

it works but once I try this code

<button id="aa" label="CORE" size = "large" onAction = "HMA_CORE"/>

and then click validate in the customUI it says that "size attribute is not declared". I am not sure what to add. I saw http://www.rondebruin.nl/win/s2/win009.htm as well, but the code looks the same. Any help will be appreciated. Thanks

The code for the buttons looks like this

<customUI xmlns="http://schemas.microsoft.com/office/2006/01/customui">
   <ribbon>
      <tabs>
         <tab id="toolRibbon" label="HMA-FUNCTIONS">
            <group id="groupDocument" label="HMA-xml outputs">
               <buttonGroup id="a">
                  <button id="aa" label="CORE" onAction = "HMA_CORE"/>
                  <button id="ab" label="PLANT" onAction = "HMA_PLANT"/>
               </buttonGroup>
            </group>
         </tab>
      </tabs>
   </ribbon>
</customUI>

Answer 1

Give this a try using built in icons from Office (get rid of buttonGroup)

<customUI xmlns="http://schemas.microsoft.com/office/2006/01/customui">
   <ribbon>
      <tabs>
         <tab id="toolRibbon" label="HMA-FUNCTIONS" insertBeforeMso="TabHome">
            <group id="groupDocument" label="HMA-xml outputs">
                  <button id="aa" label="CORE" imageMso="MacroArguments" onAction = "HMA_CORE" size="large" />
                  <button id="ab" label="PLANT" imageMso="PictureBrightnessGallery" onAction = "HMA_PLANT" size="large" />
            </group>
         </tab>
      </tabs>
   </ribbon>
</customUI>

Reference: Office 2007 Icons Gallery