c.f. http://stackoverflow.com/questions/19777515/r-shiny-mainpanel-display-style-and-font

server.R

# server.R

library("shiny")

shinyServer(

  function(input, output) {

    myFunction <- function(myinput)
      {
    
      # compute output
      myoutput <- myinput*myinput
      myoutput

      }
  
    # Show computed values
    output$myResults <- renderText({
      r <- myFunction(input$myinput)
      c("My Output:","<br><br>",r)
    })

  }#end function(input,output)
  
)#end shinyServer

ui.R

# ui.R

library("shiny")

# Define UI for slider demo application
shinyUI(

  pageWithSidebar(
   
    # Title
    headerPanel("Input-Output Shiny App")
    
    ,
    
    # User Input
    sidebarPanel(

      # customize display settings    
      tags$head(
        tags$style(type='text/css', "input { width: 80%; }")# control numericInput box
      )
      ,
      
      # user input
      numericInput("myinput", "My Input:", 0)
            
    )# end sidebarPanel
    
    ,
    
    # Output
    mainPanel(
    
      HTML('<style type="text/css">
        .row-fluid { width: 50%; }  
        .well { background-color: #99CCFF; }
        .shiny-html-output { font-size: 20px; line-height: 21px; }
      </style>')
    
      ,
       
      wellPanel(
        
        # See output$myResults in server.R
        tableOutput("myResults")
      
      )#end wellPanel
    )# end mainPanel
    
  )#end pageWithSidebar
  
)#end shinyUI