You could (theoretically speaking) just paste your skin into paint and then change its size from 128 pixels to 64 pixels. Usually, that works quite well if it's just about skins and the sizes are divideble (128:2=64). The only problem you could discover is that paint can't really work with .png, so you'd have to remove the white pixels that are automatically being added using gimp, photoshop or whatever program you have.
I could do that, but the textures of potions are connected (Means either all potions are mugs or none). However I could turn the symbol, when you have the effect active, into a cup of coffee. Which MC-version and how many pixels (original:16*16; 32*32; 64*64; 128*128; 256*256; 512*512)?