Hi,
If it is a imported Mesh, you can select the mesh in the project window.
There you can set the Scale factor and some more options

Another possibility is to make a empty parent, drop in the coin and size it the way you want it.
Then make a prefab from that.
btw if you use many coins, you might want to look into pooling.
You can find a pooler package (actions) on the
Ecosystem