Colander planting soil particle size

Hey how are you? Im planning on growing a couple of juniper and black pines pre-bonsai on colander in order to thicken the trunk. The question is what soil particles size would you say works best with the colander for this purpose: smaller or better large ones. What im planning on usong would be mainly lava and pumice. Any other recomendation would be gladly apreciated. Thanks!

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