There is a missing label in the Box tab (Width and Height options) in the StyleBuilder dialog.

You can dynamically change the label by following this example:

<telerik:RadEditor runat="server" ID="RadEditor1" OnClientCommandExecuted="OnClientCommandExecuted">
    <Tools>
        <telerik:EditorToolGroup>
            <telerik:EditorTool Name="StyleBuilder" />
        </telerik:EditorToolGroup>
    </Tools>
</telerik:RadEditor>

<script>
    function OnClientCommandExecuted(sender, args) {
        var command = args.get_commandName();

        if (command === "StyleBuilder") {
            var dialog = sender.get_dialogOpener()._dialogContainers[command];

            dialog.add_pageLoad(fixSectionName);
        }
    }

    function fixSectionName(sender, args) {
        var dialogWin = sender.get_contentFrame().contentWindow;
        var label = dialogWin.$telerik.$(".reStyleBuilderBoxSize").find("legend");
        label.html("Dimensions");
    }
</script>