0.2 mm or thicker. Fine layers are a surface-finish decision, not a strength decision, and the widespread belief that thin layers make stronger parts has it backwards.
The argument for thicker layers
An FDM part fails between layers, at the weld where one bead fused to the one below it. Thicker layers give you fewer, larger welds; thinner layers give you more, smaller ones. The total bonded cross-section is broadly similar either way.
What does change is heat. A thicker bead carries more thermal mass, so it stays hot longer against the layer beneath it and fuses more thoroughly. That is a real, if modest, argument for coarse layers on a load-bearing part.
The cost of going fine
On the 60 mm cube, dropping from 0.2 mm layers to 0.1 mm moves the job from 3.95 hours to 7.9 — an extra four hours to lay down an unchanged 57,824 mm³ of plastic.
You are paying twice over for a change that does not make the part stronger. If the part is also a display piece, that may be a fair trade; if it is a bracket, it is not.
The ratio that does constrain you
Layer height should stay under roughly four fifths of nozzle diameter. Past that, the bead cannot key into the layer below properly and bonding genuinely suffers.
That gives a practical ceiling of 0.3 mm on a 0.4 mm nozzle and 0.45 mm on a 0.6 mm one. Both are recorded in this site's nozzle reference, and both are worth respecting — this is the one layer-height rule that is really about strength.
What to change instead
If a part is failing, in order of effectiveness:
- More walls. Solid material on the skin of a part does far more work in bending than anything buried in the middle of it.
- A hotter nozzle. Layer adhesion responds directly to melt temperature, far more than to layer count.
- Print orientation. Rotating a part so the load runs along layers rather than across them changes strength by more than every setting combined.
- A different material. PETG or nylon will out-perform any PLA setting for toughness.
Infill is conspicuously absent from that list, and so is layer height.
The layer height and strength guide sets out the mechanics in full, and the print time estimator will price the change in hours for whatever you are actually printing.