Similar to the ability to set the global default icon sizes of all the items in the panel, as well as the icon sizes in the notification area, it would be good to be able to set the size of the icons in the window buttons, setting the width each of these buttons will occupy, and the padding space between the buttons. Being able to set all 3 or some of the 3 settings for the whisker menu button, launchers buttons, and notification area are also important, so potentially the solution would be to have all these settings in the panel properties dialog.
I see that the xfce4 quicklauncher plugin had 'space between launchers' https://screenshots.debian.net/screenshots/000/007/900/large.png
Size/width are determined by the two icon resolutions provided by libwnck - 16px and 32px. Padding is determined by the theme. I won't add dynamically injected CSS code and lots of appearance/layout settings - which may end up producing inconsistent results with different themes - to the plugin. If at some point libwnck can provide custom icon sizes I will obviously add this feature to the tasklist and also take the panel's icon size setting into account.
I seem to be confused that size/width is provided by libwnck and limited to 16px and 32px as can be seen in my panel screenshot. https://bugzilla.xfce.org/attachment.cgi?id=9512 My suggestion for this is because I'm designing a Windows 7 style panel and without it i cant give it a particular look irrespective of the theme.
Your screenshot seems to show launcher plugins, not the window buttons - as far as I can tell without seeing the config.
Created attachment 9554 window buttons with padding Seems i was mistaken and yes the buttons are only 16px or 32px. Attached image of what is hoped to be able to achieve with the spacing for the Windows 7 panel layout.
As I said, this is up to your Gtk theme. (launchers and window buttons without labels are the same size in e.g. Greybird)