I want a precomp of a shape layer,
but i want it to be at the right size of the shape and have the anchor point right in the horizontal and vertical center of the shape.
Whenever i take a shape and create a precomp it will immediately get the full composition height and weight and the anchor point will be in the middle of those dimensions.
How can i create a precomp of a shape layer at the right size and with the right anchor point?
A pre-comp is always going to be the same size as the original. This does not mean you cannot resize it later.
When you create your shape layer you need to press the u key twice to reveal all of the shape properties and reset the shape position To 0,0. This will center the shape layer in the composition. Now after you precompose, you can change the size of the composition.