13 inches is recommended because its the perfect length to direct energy well. Not too short, not too long.
And yes, you dont have to decorate it but decoration is also recommended for many reasons, including the following:-
It makes the appearance of the wand more pleasant.
If you are casting a particular spell, the decorations on the wand's magickal properties may correspond to that particular spell.
It wont be easily confused with other plain sticks in your house, garden or where ever you store your wands.
For example, if you was casting a love spell which involves a wand, you might want to decorate the wand with pink ribbons, hearts, etc.
Also, decoration with crystals it popular. For another example, if you was casting another love spell, you could attatch a piece of Rose Quartz or another love crystal to your want to make the spell more effective. It really works!
Oh, and make sure you use the right wood too. If you wanted a want for luck spells only, use a branch from a tree that is known for luck as one of its magickal properties! Or, if you wanted a wand for healing only, use a branch from a tree that is known for its healing power! The list goes on. If you want to know what trees are best for which, message me and i'll be happy to give you a little list.