title: Pandas pytable: how to specify min_itemsize of the elements of a MultiIndex
body: <p>I am storing a pandas dataframe as a pytable which contains a MultiIndex. </p>
  
  <p>The first level of the MultiIndex is a string corresponding to a userID. Now, most of the userIDs are 13 characters long, but some of them are 15 characters long. When I append a record containing the long userID, pytables raises an error because it is expecting a 13 characters field. </p>
  
  <pre><code>ValueError('Trying to store a string with len [15] in [user] column but\nthis column has a limit of [13]!\nConsider using min_itemsize to preset the sizes on these columns',)
  </code></pre>
  
  <p>However, I do not know how to set the attribute min_itemsize for the elements of a MultiIndex. I have tried <code>{'index': 15}</code> and it does not work...</p>
  
  <p>I know that I could force all IDs to be 15 characters long from the beginning by appending spaces, but I would prefer to avoid this if possible.</p>
  
  <p>Thank you for your help! </p>