I don't think this is possible in CRM 2013 as the DOM has been modified quite a bit and probably it's resizable property has been set to false, which is why that cannot be done. Also, I don't think there's any supported method to enable that.
A lot of the times the record list in that dialog is way to too small - normally around 5 records. I'm wondering what is the reason behind disabling the ability to resize?
Yes, I have noticed that too. The DOM and look and feel of CRM 2013 has gone a substantial change since CRM 4.0; and I believe it's due to the design that the resizing ability has been disabled.