If you wish to draw very fine lines, you’ll want to go for a 0.3mm lead. For writing and most drawing, you’ll probably prefer a 0.5mm lead. A thicker, 0.7mm lead, meanwhile, may be better for sketching and non-detailed drawing. Other sizes are also available for specialist uses.
It's not just about size, though: also consider the hardness of the lead. A soft lead will be darker and more prone to smudging, while a hard lead is better for leaving light, fine lines. Softness is represented by a number followed by ‘B’; the higher the number, the softer the lead. Hardness is represented by a number followed by ‘H’; the higher the number, the harder the lead. Finally, ‘F’ and ‘HB’ leads lie in the middle..