"Change Icon" dialog is available in Customization dialog's Toolbars tab, Assigned Commands list (right-click an item, or select an item and press the "Modify" button below).

There are (currently) three icon sizes for toolbars, selectable from Options->View: small (16x16), large (24x24), and extra large (32x32). All three may be assigned to a command on a toolbar; but the UI allows to do it only in a unobvious way: one needs to set the toolbar icon size to small in Options, then assign small icons; then set the toolbar icons to large, then set the large icons; and finally, set the toolbar icons to extra large, then set the extra large icons. And this means, that when you call the "Change Icon" dialog, it works with the currently selected icon size.

The user who assigns the icons, doesn't see any hint that would tell them all this. They will usually not realize, that the icons that they assign will only be shown when the toolbar is shown with the same icon size (that e.g. means that if a user changes their icon size preference, or if the toolbar stored in the document is opened on another system with different icon size, the icons will unexpectedly disappear from the buttons, and only the names will show).

The dialog needs a way to define all three icon sizes. A possible solution could be to add tabs to the dialog for each size. This would allow *both* to simplify the icon assignment workflow, and to inform the user about this in the first place.