In the classic pentagon all edges have the same length and therefore all internal angles are the same (measuring 72 °). Pentagons are often used when creating 3D objects like toy balls or bowls (watch the pattern category, we are planning to put together pattern packs for these).
 
We define the size of the pentagons by the length of the edge, as indicated in the diagram.  Please be aware that Pentagons do not tessellate with Hexagons in ‘flat’ patterns! They are best combined with 36° and 72° Diamonds.