The below should help you. Also, CSS is good to know about but probably not necessary for what you are trying to accomplish. setting the width of "selectInput" to a percent value, it will take up that percent of the enclosing element (in this case "column" which in turn is enclosed and width ultimately limited by the "sidebarPanel").
library(shiny)
ui <- fluidPage(
sidebarLayout(
sidebarPanel(
fluidRow(
column(width = 12, offset = 0,
setwd("~/Development/Dir/OUT/openMS/INPUT_DATA/"),
selectInput('selectfile','Select File', choice = list.files(),multiple = TRUE, width = "100%"),
p('You have selected: ', verbatimTextOutput('fileselected')), # p represents a HTML <p> element
setwd("~/Development/Dir")
)
)
),
mainPanel(
)
)
)
server <- function(input, output){
output$fileselected <- renderPrint({ # Change renderText to renderPrint
# make each selection print on a new line
cat(input$selectfile, sep="\n")
})
}
shinyApp(ui = ui, server = server)