Set layer mask
Layer mask looks like an erasing effect and the content is not resized. It is set for a layer and only affects the content used in this layer without changing the native content.
In 
Screens / Aux.⚓
Select a layer.
In Properties, click Mask to show the corresponding settings.
Set the masking per side in percentage or pixels.
Note :
When multiple layers are selected, the crop is set on the following layers in percentage so the ratio stays the same even with content with different resolutions.