How would you create a flashlight effect?

You can feather a mask path.

You cannot feather a shape layer path, but you can add a mask inside a shape layer and feather the mask in the shape layer.

Editing a mask is fairly straightforward. Holding down the shift key with the Shape tool (q pressed enough times to bring up the ellipse tool) selected will constrain the mask or shape to a circle, but it will only appear to be round if your composition is set to square pixels and in this day and age you should not be creating any comps that are not square pixels and standard frame size. If you don't know how to use the mask tools it means you have not spent any time with the User Guide or the online manual. Pressing 'f' will bring up the mask feather tool or you can presss 'g' twice to bring up the Mask Feather tool so you can add different amounts of Feather on different parts of the mask. 

With the mask selected in the timeline and the selection tool active (v) you will see the cursor change from black to a white arrow while you move around the comp panel. You can drag a selection around a single vertex, or multiple points, or you can double click on the mask to bring up the mask transform tool to edit the mask.  With the pen tool selected you can hold down the Alt/Option key to change the pen to the convert point tool so you can create or adjust the bezier handles to adjusting the tension on the points if you have drawn a Rotobezier mask. 

Creating an adjustment layer with a mask is one approach. Another is to use a shape layer with a white fill and the blend mode set to Add or Screen and adjust the opacity. Another method is to make the layer 3D and add an ambient light with a brightness of about 50% to the scene and a spotlight with a brightness of 100%. You can also use a plugin from Trapcide called LUX to add atmospheric lights to your comp. There are at least 20 or 30 ways I can think of to simulate a flashlight in a scene that runs all the way from so realistic you would never believe it is an effect to a cartoon. 

If we had any idea what your comp looked like or what your design goals were we could point you to the most efficient workflow. 

Maybe a different approach would be to use and After Effects spot light.

Make sure that the picture is in 3d space.

Go to layer/new/light.

I would also turn off the point of interest by going to layer/transform/auto orient/ set to none.

Then keyframe the position of the light.

Without any exact info about your comp and such nobody can tell you much. simply sounds like you created it at an unusual aspect ratio or have messed up things with a layer switch liek 3D.

Mylenium