You follow the power of two rule when you operate with textures -> basically any of the following dimensions ("8", "16", "32", "64", "128", "256", "512", "1024", "2048") as canvas for your texture (filled in with whatever background filler you need - white, black etc)
If you are working with photoshop, create a new canvas with the above dimensions that would fit your actual texture inside. (unless your texture have a dropped shadow or an outter glow - the color of the canvas doesn't matter - you can use white/black whatever). Then you need to create an Alpha Channel that would isolate your texture from the rest of the image. Locate Channels tab, create new channel, make selection of your actual texture and paint that selection within the alpha channel with white brush. Save as dds/tga (bends on lower details video settings, has lower size / raw input with extra colors but bigger in size)
When you import that texture, make sure Alpha channel is selected/enabled on it (newer UEditors have it during the import screen, older version you have to go through properties/surface)