Feito meio à pressa, mas funfa...
Public Sub MostraTamanhoCampo()
Dim dbs As Database
Dim tdf As TableDef
Set dbs = CurrentDb
Dim fld As Field
Dim x As String
Set tdf = dbs("SuaTabela")
For Each fld In tdf.Fields
If fld.Name = "SeuCampo" Then
x = fld.Size
End If
Next fld
Debug.Print x
dbs.Close
End Sub