One way to do that is to use the "Placeholder" in PowerPoint.
- Use View → Slide Master, then choose your layout.
- Select "Insert Placeholder" and choose Picture.
- Use the cursor to drag the picture to size and position it.
You can make multiple placeholders, but you do not have to use all of them if a slide does not require it.
From now on all new slides (with the chosen layout) will have the placeholders where you can insert a picture and they all will be cropped to auto fit the placeholder, or auto fill.
What you are trying to do is not possible. The Crop to fill will take the original and without shrinking it will use the center part and fill it in the preassigned Place Holder. The crop to fit will have to reduce the original to show it as whole in preassigned place holder.
Here is an example (left and right are place holders) while the middle is the original. So even if using the crop to fit as default the image size will be what your place holder was. Thus using the Crop to Fill is better option while using the Crop tool to move to the part of the image you wanted to show in full size.
The problem with your request is that the Crop to Fit will always require adjustment depending on the original image size.