High-res to Low-res

Which percent recommend me to reduce a texture if i want to convert it from high-res to low res???
If a texture is 1024 X 1024 it would be fine to convert it to 128 X 128??

If a texture is too small (for example 32 X 32) it doesn't need to be resized, no??

no, it will be crap if you use a texture with 128x128. You can reduce a texture to 512x512 and it will be still ok, but that's the minimum to get a good result ,i think.