I make 2D games with Python and Pygame. I usually make my window size 640x480, and use tiles that are 32x32. My questions is what size for the window and tiles should I be using? Is there an advantage to using one size vs another (say 500x500 instead of 640x480)? What are the considerations if my game can be played at full screen or at varying screen sizes? 

Most modern monitors are 1920×1080 or a multiple thereof.  Therefore, if you want to support full screen at all, you should use 1920×1080 or a fraction thereof as your base resolution, because that would allow you to hit full screen by scaling scaling by an integer factor.  Your best window sizes are therefore 640×360, 960×540, and 1920×1080.  If you pick 640×360, also allow a window size of 1280×720 with a scaling factor of 2.