UV1 (or better, channel 0) is normally used for texture mapping, and not for lightmapping. The size of the UV1 layout is relative just to the size of the textures and materials you will apply (e.g., the smaller the layout is, the larger the textures will appear on it)
UV2 (channel 1) is used for lightmapping. In case it is useful to optimize the layout in order to fill most of the space available, since all the space around the objects will waste memory in unused parts of the lightmaps. Checkout page for an in depth explanation: UDK: Lightmapping Basics and 18 Important Principles for Creating and Using Lightmaps for UDK
(the article talks about UDK, but it is totally relevant to UE4)