Amateur problem - How can create you create an exact measurement using Pen, Shapes etc.

Andrew's got the right method for you, but there's also another option which you can use when your solid is not rectangular.

• Draw a shape path or mask.  Any shape, so long as the number of points corresponds to the number of point/corners you want your solid to have.
• Expand the shape on the timeline until the path is visible and select it
• Go to Window ... Create Nulls from Paths.jsx
• In the pop-up choose, Points follow Nulls

AE with create null objects for each point and link the path points directly to these nulls.  So you can set the position property of the null and the path will follow.  The benefit of this approach is you can keyframe the null positions to change the shape.

Easy solution.  Create a solid that is 1920x1080.  Then double click on the rectangular mask tool in the toolbar.  This will automatically create a mask that is the exact size of the 1920x1080 solid layer. 

Then create a second solid layer, but press the Comp Size button so the new solid is the size of your expanded comp.  Copy the mask from the first solid and paste it into the second solid.  You can delete the original solid if you like. You now have a comp-sized layer with a 1920x1080 mask applied.

Follow the original instructions now: set the mask to Subtract, lower the opacity, and set the layer as a Guide Layer.

Nope, transform values are just properties not specifically tied to an object. If you want to "lock" them, you have to create stuff with scripts or use animation presets that you have created with stuff measured to a T.

Mylenium

I realised you can create the solid shape from the beginning with a specific measurement which is great. But i am still curious if there is such feature that enables you to create shapes with specific measurements