The group chat (GC) room roster in Microsoft Skype for Business 2015 (Lync 2013) currently opens at 200px. However, it should open at 100px as per UBS requirements. Additionally, if you drag the GC room to a new position, it should stay at the new size for all the other rooms.
To fix this issue, install the February 7, 2017, update (KB3161988) for Skype for Business.
Note Lync 2013 was upgraded to Skype for Business in April 2015.