How to verify size of .ply model uploaded into Slicer?

You can verify the size/scale by measuring a known distance on the model using markups Line tool on the toolbar:

If the measured size does not match what you expect then use the ratio as scaling factor. For scaling, you can use “Surface Toolbox” module (a bit simpler to use than Transforms module, as it applies and hardens the transforms to a new model in one step).