How to sort data and get new coloum number for it

You are accessing the row at index 4, which is the 5th row in the DataTable which is why you get the vale of '150. Datatable indexes start from 0 -

Dim size1 As String = table.Rows(3).Item("Size")

If you want to sort the DataTable by the 'Size' column in ascending order before accessing the rows -

table.DefaultView.Sort = "Size ASC"

Your code should then look like -

Public Class Form1
    Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
        Dim table As DataTable = New DataTable("Players")

        'Add 2 columns...
        table.Columns.Add(New DataColumn("Size", GetType(Integer)))
        table.Columns.Add(New DataColumn("Team", GetType(Char)))

        'Add 5 rows...
        table.Rows.Add(100, "a"c)
        table.Rows.Add(235, "a"c)
        table.Rows.Add(250, "b"c)
        table.Rows.Add(310, "b"c)
        table.Rows.Add(150, "b"c)

        'Sort your DataTable by the 'size' column in ascending order...
        table.DefaultView.Sort = "Size ASC"

        'Get the value from the 4th row (index 3)...
        Dim size1 As String = table.Rows(3).Item("Size")

        'Convert the DataView back to a DataTable...
        table = table.DefaultView.ToTable()

        MsgBox(size1) 'Output should now be 310...
    End Sub
End Class