Exactly as LanFanNinja said in above comment you are passing String Value where you have to pass Single.I think you are using Vb.Net languge
The syntax to Set Font for RichTextBox is as follows :
RichTextBox1.SelectionFont = New Font("Tahoma", 12, FontStyle.Bold)
You change your code and try this :
RichTextBox1.SelectionFont = New System.Drawing.Font(ComboBox1.Text, CType(ComboBox2.Text, Single), RichTextBox1.SelectionFont.Style)