I’m thinking about two different projects:
– Low, narrow bed for my son. I’m considering whether to keep the twin’s 190cm length, or to also make it shorter. I want to keep this simple, building the frame as a big dovetailed box with arches on all four sides. Slats would rest on internal rebates.
– Long shelf, about 150cm, between two kitchen cabinets, in front of my window. I’d also make this starting from a dovetailed box, so the sides can be screwed into the cabinets on both sides, and arched front and back. These shelves wouldn’t support nearly as much weight as my kid, but it would have some plants and a dish and pan drying rack right in the middle.

Both of these projects are based on a dovetail box, are very long, and would support some weight on top. How thick and wide would the boards that make the box need to be in each case to avoid distorting the long part of the frame?

Thanks!