Currently the default height of RadButton with Silk skin and CssClass="rbPrimaryButton" is 28px. The correct height must be auto, so that the text inside the RadButton can be wrapped when it is longer and no height is specified. For the time being the following CSS workaround can be used:
CSS:
        span.RadButton.RadButton_Silk.rbLinkButton.rbPrimaryButton {
            height: auto;
        }
ASPX:

            <telerik:RadButton ID="RadButton1" runat="server" Skin="Silk" ButtonType="SkinnedButton" Text="aaaaaaaaaaa bbbbbbbbbbbbbb cccccccccccc" Width="200px" CssClass="rbPrimaryButton" />
            <telerik:RadButton ID="RadButton3" runat="server" Skin="Silk" ButtonType="SkinnedButton" Text="aaaaaaaaaaa bbbbbbbbbbbbbb cccccccccccc" Width="200px" CssClass="rbSecondaryButton" />