A short tutorial I wanted to do for a bit now. How to improve the texture quality of MMD models.
Lets say you have a gamerip model like this (Doesn't have to be Pokemon, I just chose this) or just a texture thats dimensions are small when you add that texture to a model usually it will appear rather pixelated like the before images because the dimensions of the image are small. The original dimensions for this models textures were 256x256. Now the After images textures are 1024x1024!! Making the models textures sharper and cleaner!
Doing this is VERY easy. All you have to do is go to this site waifu2x.udp.jp/ and upload your image. For the settings choose "Medium" for noise reduction and for up scaling choose "2x"
I suggest you run it through twice for a better quality image if your original picture was small but do not do it more then that because the image will start to get a little muddy/blurry, you don't want that.
The site basically upscales your image, cleans up any pixealted parts and smooths it out.
AND THAT'S IT! Enjoy your higher quality models, this can work with literally any model and you don't need to adjust the UV once you're done.
Alternatively, you could just get the UVMapping plugin for PMX/PMD Editor(s) and scale up the mapping. That would scale down the textures on the model, but it requires accuracy. ...and lots, lots of updates... xD
Not sure why it didn't work for you? What format did you try to enlarge? Upscaling an image regularly doesn't do much because and detailed textures will be rather blurry. Doing this it upscales it, sharpens and smooths out the image making the details of the smaller image stay intact.