You mean 32 pixels? Or 32-bit?
The pixel size for your image should reflect your Project Window size (found on the left after clicking your project name on the top right). This is the pixel size of the window. But: Construct 2 can upscale the resolution of images after rezising the window in fullscreen mode. That means, if you want your game to run on PC high end machienes (1080p), you should adjust your sprites to that size and scale them down within Construct 2 (because C2 will use the full resolution image in full screen then instead of upscaling your small image).
Approximately for 1080p:
- Big sprites: 900-2000px
- Medium Sprites: 200-500px
- Small sprites: 50-100px
You can use even way lower values if you want to go to mobile only (you even should and have to!)
Hope this helps somewhat.