The ComboBox and TextBox appear with different size/font and layout settings because of their built-in themes. There is a workaround for this, however I suggest you to wait for couple of weeks and try out the upcoming release where we have corrected these issues.
Here is the simplest workaround:
this.radComboBox1.ComboBoxElement.TextBoxElement.Padding = new Padding(2, 2, 2, 3);
Let me know if I can help you any further with this.
the Telerik team