LittleTiles

This mod allows you to build anything you want. It adds a way to add more detail to everything. You can hammer blocks into tiles which can be 4096 (or even more) times smaller than an ordinary minecraft block. You can combine your tiles together to create doors, chairs, ladders, no-clip, storage and any kind of furniture structure.

Pehkui is mod allows for most entities to have their size and other properties changed, either enlarging or shrinking them.(This mod goes well with LittleTiles.)

WorldEdit

A Minecraft Map Editor... that runs in-game!
With selections, schematics, copy and paste, brushes, and scripting.
Use it in creative, or use it temporarily in survival.

Axiom is an all-in-one tool for Minecraft editing. It merges the capabilities of 3D software with a sleek interface and an innovative toolset. Axiom simplifies Minecraft building while elevating its potential. Real-time previews and client-side rendering, powered by specialized algorithms, enhance both visual feedback and performance while building.

By installing Axiom, you agree to the EULA available at https://axiom.moulberry.com/eula

LittleTiles + WorldEdit/Axiom (Forge)