Don't set the Height of the ToolBar or the Button instead try setting the Height of the Image control.
<Button Command="Test" ToolTip="Test">
<StackPanel Orientation="Horizontal">
<Image Source="/Samples;component/Images/test.png" Height="30" Width="30" />
<TextBlock Margin="3,0,0,0">Test</TextBlock>
</StackPanel>
</Button>
Samples - ToolBarControl