Highlights d Microtubule nucleation scales spindles in zebrafish embryos and Xenopus egg extracts d Microtubule nucleation is regulated by the surface of the cell d Microtubule polymerization only contributes to scaling below a critical cell size d Microtubule number and length are regulated using distinct geometric cues

Precise sample orientation is crucial for microscopy but is often performed with macroscopic tools and low accuracy. In vivo imaging of growing and developing samples even requires dynamic adaptation of the sample orientation to continuously achieve optimal imaging. Here, we present a method for freely positioning a sample in 3D by introducing magnetic beads and applying a magnetic field. We demonstrate magnetic orientation of fixed mouse embryos and artemia, and live zebrafish embryos and larvae on an epi-fluorescence microscope and on a light-sheet system for optimal imaging.