How do I resize a video layer (not the compossition) in After Effects?
Assuming you meant the composition is 1920x1080, and you wish to resize a layer to match the composition size, do one of the following: 

Right-click on the layer name. In the menu that comes up, click on Transform --> Fit to comp.

Another method is, in the controls for Scale, under the Transform drilldown, right-click on one of the values. Click on Edit Value.  Change units to % of composition. Alter the value of the larger dimension, out of Width and Height, to 100%.