Ink spreads a little under the skin over the years. Lines drawn too close together slowly join, so a tiny design with lots of detail turns into a blur.
As a rule of thumb, keep gaps between lines at least 1 mm and lettering at least 5 mm tall. A 3 cm piece works for a simple symbol; a detailed animal needs 8 cm or more.
Hold a ruler against the place you have in mind, or look at our flash sheet: every piece on it is drawn at its real size on a centimetre grid.