width.R

library(shiny)

predict <- function(foo_or_bar) { ifelse(foo_or_bar, 'Foo', 'Bar') } 
style <- "
    /*
        Every textInput is wrapped with div with following classes
        Maximum width is by defult 595px. We can change it to 100% 
        of the parent element
    */

    .form-group.shiny-input-container {
        width: 100%
    }

    /* # matches id of the element -> inputId from textInput */
    #foo25  {
        width: 25%  
    }

    #foo50  {
        width: 50%  
    }

    #foo75  {
        width: 75%  
    }
"

runApp(shinyApp(
    ui = shinyUI(fluidPage(
        tags$head(tags$style(type="text/css", style)),
        fluidRow(
            column(
                # Column 12 can use a whole width of the page
                width = 12,
                textInput('foo25', 'I am foo25 inside column12'),
                textInput('foo50', 'I am foo50 inside column12'),
                textInput('foo75', 'I am foo75 inside column12'),
                textInput('foo', 'I am some other foo inside column12')
            ),
            column(width = 9,
                # Width of the child is limited by the width of the parent element
                textInput('bar', 'I am some bar  inside column9')
            ),
            column(width = 5,
                # Same as above
                textInput('fobar', 'I am some foobar  inside column5')
            )
        )
    )),
    
    server = shinyServer(function(input, output, session) {
    })
))