Here in this post i am going to give to some tips related to small size graphics design in photoshop.
First thing is that always use the square pixel in size, means the height and width of the canvas should be same.
You could use the large canvas size during the design like 200×200 pixels during designing process then resize it according to your need at the end.
Second important thing is that always use the higher dpi. I usually use 300.
If you are planning to design the icon with alpha channel means background should be transparent then use Transparent in the background options.
I would suggest not to use the Text on Icons but if its really necessary then always set the Anti-Aliasing option to None.
If you are designing the really small icon of the size 20×20 or 15×15 then try not to use too much colors there because its really mess up when u resize it.
I would always prefer to use the 1px darker color border outside the shape to be used as icon or use a inner shadow to make it more prominent than that of background on which it would be used later on.
At the end when you are finished all the design and its ready to go then go to image>>Image size, In the dialog box make sure that constrain proportion and Resample image are checked, use the Bicubic in Resampling method.
Thats all, save the image in your desired format….either gif,Png, or anyother.