from SO question 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$myResults <- renderTable(
      {
        r <- c("My Output:",myFunction(input$myinput))
        myTable <- do.call(rbind, list(r))
        myTable
      }
      , align = c("clc")
      , include.rownames = FALSE
      , include.colnames = FALSE
      , sanitize.text.function = function(x) x
    )#end renderTable 
  }#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 in sidebar panel
    sidebarPanel(
      # customize display settings    
      tags$head(
        tags$style(type='text/css', "input { width: 100px; }")# control numericInput box
        ,
        tags$style(type='text/css', "select,textarea,.jslider,.well { font-size:50%; } ")#  control font / ineffective
      )
      ,
      # user input
      numericInput("myinput", "My Input:", 0)
    )# end sidebarPanel
,
    # Output in main panel
    # See output$myResults in server.R
    mainPanel(
      tableOutput("myResults")
    )# end mainPanel
  )#end pageWithSidebar
)#end shinyUI