Put the progress bar in the center and change the size of the message box in R Shiny

I am writng a R shiny web application. I am looking for a way to put the progress bar in the center of the webpage and adjust the size the of the bar. The function I used is "withProgress", but I have the trouble to find the parameters for the location and size. Any suggestions?

withProgress(
      message = 'Running ......',
      detail = 'This may take a while.',
      value = 0,
      {
        ......
      }
    )

Target the progress bar container (the notification panel) with some CSS:

#shiny-notification-panel {
    /* Position bottom-right corner in the center of the screen. */
    position: fixed;
    bottom: 50%;
    right: 50%;
    /* Shift the position to center alignment. */
    transform: translate(50%, 50%);
    width: 700px;
}

In an app:

library(shiny)

ui <- fluidPage(
  tags$style(HTML(r"(
    #shiny-notification-panel {
      position: fixed;
      bottom: 50%;
      right: 50%;
      transform: translate(50%, 50%);
      width: 700px;
    }
  )")),
  titlePanel("Progress"),
  textOutput("progress"),
)

server <- function(input, output, session) {
  output$progress <- renderText({
    withProgress(
      message = 'Processing...',
      detail = 'This may take a while...',
      value = 0,
      Sys.sleep(1000)
    )
    "Done!"
  })
}

shinyApp(ui, server)