Private Sub ComboBox1_Change()
Application.EnableEvents = False
ActiveCell.Value = Me.ComboBox1.Value
Application.EnableEvents = True
End Sub

Private Sub Worksheet_SelectionChange(ByVal Target As Range)
If Intersect(Target, [e1:e1000]) Is Nothing Or Target.Cells.Count > 1 Then HideCombo: Exit Sub
Application.EnableEvents = False
Me.ComboBox1.Top = Target.Top
Me.ComboBox1.Left = Target.Left
Me.ComboBox1.Width = Target.Width + 16
Me.ComboBox1.Height = Target.Height
FillCombo
Application.EnableEvents = True
End Sub

Sub FillCombo()
Const iRow = 65536: iClm = "A"
iRws = Sheets(1).Columns(iClm).Rows(iRow).End(xlUp).Row
ComboBox1.ListFillRange = "A1:A" & iRws
Me.ComboBox1.Value = ActiveCell.Value
Me.ComboBox1.Font.Size = 6
End Sub

Sub HideCombo()
Me.ComboBox1.Top = 0
Me.ComboBox1.Left = 0
Me.ComboBox1.Width = 0
Me.ComboBox1.Height = 0
End Sub