A power of 2 resolution for bother width and height is highly recommended since this allows mip maps to be created (so if you have a 2048x2048 texture, the engine will stream a 1024x1024, 512x512, 256x256, 128x128, 64x64… texture based on the view distance and screen size). 2048 for mid scale objects is as far as my understanding goes the industry standard nowadays (on PC at least), 4096 is great, and 8192 is a bit too much (BTW the max texture resolution that will get streamed is 4096 unless you edit the Engine.ini). For the size of the texture I recommend trying some and play with it to see which resolution is optimal.