what tends to be the usual dimensions you use for creating sprites, and a followup question how would you add small details to sprites would I have to use large dimensions or is there another way. Thanks in advance for the help 
I think you mean Pixel?
As for sizing, there is no real standard; use what works for your project based on the target resolution. Powers of 2 seem to be popular 32x32, 48x48, 64x64.
As for the question about small details, that is likely part of the creation-pipeline. When you make your sprites, add in the details then. Otherwise, you can use the Unreal material system to layer textures/effects on top of/into your base sprite.