vitorfgd, that is a very complicated subject and you need to be sure of exactly what it is that you want to achieve.
1. Do you want the sprite to take up exactly the number of pixels on the screen irrespective of screen resolution? For example, a 128x128 sprite will look ok on an 800x480 screen but it would look really small on a full HD 1920x1080 screen - and the apparent difference in size would be very striking if the screen dimensions were about the same.
2. Do you want the sprite to take up the same proportion of the viewable screen length/width irrespective of screen resolution or aspect ratio? C2 handles this option very well with its Scale Outer scaling option (and the others, but they are less on mobile usable IMO).
The common advice is to draw your game for the largest aspect ratio you can expect (16:9 or 16:10) and then ensure that your background fits smaller aspect ratios to good effect. There are some great tutorials and threads on this, so do a quick search and have a look through. It's not a decision you can just glibly ignore at the start and then hope to fix later in your development. It also depends on your export target - I understand that C2 does scaling very well so, providing the sprites have the detail, you could draw all of your sprites and layout for 800x480 for example, with each sprite drawn at 1/2 scale, so that if there is any upscaling to be done (full HD / retina screen) then the upscale of your sprites can be done by adding that detail rather than by interpolating pixels. Again, plenty of tutorials ect will cover this better than I can. Personally I have scaled my last few apps to a middle of the road aspect ratio that is better suited for iOS - something like 960x640 or 1136x640 - both are ok for android and iOS and the scale outer can make some exports look longer and some look wider, but I'm personally happy with the average appearance. Whatever works for you...