… after updating and setting new dimensions for thumbs (grid layout) there seem to be myriads of auto-generated thumbnails in media-(upload-)folders!

due to the fact that this increases data amount on webspace rapidly i want to get rid of all those thumbnails that wont be used anymore…
can this be done without any danger of big chrash?

first problem: how do i get to know which one is actually used?
second problem: which is the plugin of choice to cleanup all these?

thx in advance