title: Pandas groupby: get size of a group knowing its id (from .grouper.group_info[0])
body: <p>In the following snippet <code>data</code> is a <code>pandas.DataFrame</code> and <code>indices</code> is a set of columns of the <code>data</code>. After grouping the data with <code>groupby</code> I am interested in the ids of the groups, but only those with a size greater than a threshold (say: 3).</p>
  
  <pre><code>group_ids=data.groupby(list(data.columns[list(indices)])).grouper.group_info[0]
  </code></pre>
  
  <p>Now, how can I find which group has a size greater than or equal 3 knowing the id of the group? I only want ids of groups with a certain size.</p>
  
  <pre><code>#TODO: filter out ids from group_ids which correspond to groups with sizes &lt; 3 
  </code></pre>