Align irregular layers to a grid with padding

Hello,
I developped a script , IZI BENTO GRID, it is great (in my opinion 🙂 ) to create and manage grid like your attached exemple.
You can find it on Gumroad ...

In your example, there are four layer sizes and the sizes have a very specific relationship to each other. For example, if the small squares are 100x100 and the gap between is 15, the rectangles have to be 100x215 (or 215x100) and the large squares would have to be 215x215. Unless your layer sizes are related like that (and it doesn't appear that they are), there's no algortihm that can assemble such a grid without scaling (and probably non-uniform scaling) the layers.

Don't discount using expressions. You can get any layer size using sourceRectAtTime() or thisLayer.size.

By default, the Anchor Point is always at the layer's center. To distribute layers in a grid, you must know how wide the biggest layer is and then set up a multiplier based on the layer's index. 

I don't have time to fiddle with a grid, but here's an expression that offsets layers in Y every 800 pixels starting with the position of a null layer on top of the stack.

ref = thisComp.layer("Null 1");
ofstNo = ref.index + 1 - index;
grid = [800, 800] / 2; // pad + 1/2 the size of largest layer
ref.position + [0, - ofstNo * grid[1]]

It won't take much to take this starting point and set up a grid that is 10 X 15 and has all layers below lined up without doing anything else. You can then animate the position of the Null to move the entire grid around. 

This stacks different-sized layers up in Y with 20 pixels between them;

ref = thisComp.layer(index - 1);
lyrHeight = height/2 + 20; // 20 pixel padding
newY = ref.position[1] + ref.height/2 + lyrHeight;
[ref.position[0], newY]