How can i remove small pixels in photoshop?

What’s with the colors – are they relevant? 

Can you provide a file? 

Another approach would be Topaz Simplify.  I have no idea what it costs now (I have owned mine for 20 years).

@Gökhan305670307wv8 – I agree with @Semaphoric dupe the image layer, run the Filter > Other > Minimum at a value such as 12px or whatever is required. This will destroy unwanted and wanted detail. Then add a black layer mask hiding all content. Then with a solid white brush with a hard edge (perhaps a square brush tip), paint over the areas that you wish to remove. The mask will then only show the filtered areas where you have painted.

I would try the Maximum or Minimum filters.