Mine

PangoContext         *pContext = gtk_widget_get_pango_context( GTK_WIDGET(pTextView) );
PangoFontDescription *pFont    = pango_context_get_font_description( pContext );
gint                  iSize    = pango_font_description_get_size( pFont );

pango_font_description_set_size( pFont, iSize + PANGO_SCALE );
gtk_widget_modify_font( GTK_WIDGET(pTextView), pFont );