Hey 
…I saw you got no response on this so thought i try and help seeing Ive got the same problem as you. This link will show you how to pack the materials into a smaller material but still keeping the image quality Packing CR and NOH maps with GIMP - YouTube. PS. change the play speed the vid is fast. Then you follow this guys stuff Landscape Textures - Building Worlds In Unreal - Episode 3 - YouTube . He has a hole series of stuff showing you everything you need to know and easy to follow. Enjoy and good luck.