Thanks, tylerdurden, this is exactly what was happening, I just corrected the stroke width and everything is fine.
Tnx!

Hi, I'm new to Inkscape and been trying to figure out the simple things, like dealing with shapes. When creating a rectangle, I've been setting its side's lenghts in the Tool Bar , writing W and H values. I just created a 10 X 10 px square this way, but when I selected it (with the Selection Tool ),...