First of all, this doesn't belong here. Questions like these should go here:
http://www.pokecommunity.com/showthread.php?t=237894&page=5
Secondly, under no circumstances should you resize a sprite. They are meant to be small as a rule of thumb. Resizing it distorts the image, so don't do that. If people want to view your sprite closer, they have the ability to copy, paste, and zoom in on your image using an image program on their computer.
Third, I would suggest using a simpler paint program that is already preinstalled on your computer. Paint.net sounds like something you use for making pictures and not sprites. You can usually find the paint program under the "Accessories" folder. Use the pencil tool and not the paintbrush. Hope this helps.