If you're using Blender to make models for UDK/Unreal but having trouble getting the size of your object right, a Blender 2.57(+) file is now available from KatsBits tools section. The *.blend contains a series of primitives blocks of varying size, useful for when modelling levels in Blender and/or 'world' ('map') objects. The Blender file has also had its grid setting sorted out and ready for use with UDK et-al. Pack includes 128x128, 256x256, 512x512 and 1024x1024, just open the file in Blender 2.57(+) and away you go. Download the UDK ref blocks here.
Incidentally for those interested, UDK uses the same sizes and grid settings as idTech so technically models are interchangeable between the two technologies - a 256x256 brush in Radiant is the same size in UDK Editor and visa-versa. The reference blocks pack will also be useful for anyone developing content for or on similar "BSP" based or type game engine tech.