Minetest only supports a limited set of formats.
OBJ can be used for static models, but for animations you will want to use Blitz3D's format (b3d).
Currently broken in Blender v3.0, use v2.8/v2.9 for now!
Make sure you have your mesh object selected, or it will not export correctly.
1 node = 1m³
However, models are all at 1/10th scale. If your model is 3m wide, it will appear as 0.3m.
In Blender go to "Scene Properties" > "Units", set "Unit System" to Metric and "Unit Scale" to 0.1.
Blender will now display units as 1/10th their actual value, so they match with in-game display.
This does not affect the actual data though!
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