How to reduce layer size along with composition

If you need to resize the composition there is a script in the Demo Palette under the File menu that will do that. 

A more efficient way to solve your problem would be to just create a new composition that is the size and frame rate and length you want and then drag the original comp to the timeline, then use the Layer/Transform menu to fit the nested comp to the current comp width. Turn on collapse transformations if you are scaling up and it will preserve the detail for vector and text layers. CT may also help if you are scaling down the nested comp.

There may be design constraints you want to consider if your comp contains a lot of small text or fine lines that move, but for most projects just dropping an existing comp in another comp that is a different size, then scaling the nested comp is all that is required.

1.- Change the size of your composition to 1280x720.

2.- Go to Layer > New > Null Object

3.- Use the parenting colum in the timeline panel to parent all your layers to the null object

4.- Change the scale of your null object. All your layers will be scaled maintaining all their properites relative to the null.

You can also scale the filez size when rendering. On the output module settings, you can render your composition to 1280x720. Of course you can use Adobe Media Encoder to do it.