In the screenshot above I set the BoxComponent size by setting the Box Extent to [128, 32, 16] (found under the Shape category when the component is selected). The StaticMesh I used was /Engine/EngineMeshes/Cube, which is 256x256x256. To match the collision component, I set the Scale of the mesh to [1.0, 0.25, 0.125]. I’m a relatively new user of Unreal, so maybe I’m not showing you the preferred way, but the above worked for me in prototyping my own WIP 2D game. (I eventually replace any explicit collision components created within the editor with static mesh assets that include custom convex collision primitives.)