See this code

Private Sub Command1_Click()
    ctrlName = "DynTxtBx"
    Set txtObject = Form1.Controls.Add("VB.TextBox", ctrlName)
  With txtObject
    .Visible = True
    .Left = 250
    .Width = 4000
    .Height = 250
    .Top = tValue
    .Text = "This is a dynamically created TextBox No." & CStr(cIndx)
  End With
End Sub

Or you can use this also

Dim c As Control
c = New TextBox()
c.Name = "myControl"
c.Width=200;
c.Height=200;
Me.Controls.Add(c)

Try this code.

You can change the size using WIDHT and HIGHT property of textbox like;

textBox.Height=200;

textBox.Widht=200;

Best Luck!!!!!!!!!!!!!!!
Sujit.