Why can't I size down my models?

Try to place the model to another position size down it and place it back to the position where you want it.

From my knowledge, Roblox isn't very good at sizing down grouped meshes, even if its with a plugin, i recommend ungrouping the model, sizing it down and then grouping it together again. If that doesn't work, try sizing down the whole model using the "properties" panel.

If your model splits apart, then try undoing it and trying it again. That's what works for me. (P.S. sometimes if you size something to low it tends to split apart so try to do small portions at a time if your scaling a big model into a small one.)