If you choose 'woody plant', you will select trees, woody climbers, and shrubs which are >50 cm tall:

This option includes trees with a well-developed main trunk...:

...shrubs which are taller than 50 cm at maturity...:

...and different types of woody climbers:

By choosing 'herbaceous plant or small shrub' you will select herbaceous plants of any size...

...plus small shrubs (those which are lower than 50 cm at maturity):

Since the height of plants is variable and conditioned by several ecological factors, many shrubs will appear under both options.