Post Heights in Grid Format

I’ve been trying to figure this out but to no avail.

I have my page set up as 2 full width posts followed by the grid and I can’t seem to set a static height for the grid posts.

I have one Standard and one Video post at the moment in grid section and it seems as though the embedded video thumbnail is smaller than the standard post image thumbnail.

I’ve tried editing the css but it then effects the two full width posts.

Any tips ? 🙂