Manufactured by Hitachi - so good quality - these are the medium and most popular size of clamp on ferrite sleeve.

They work by being clipped on to cables that are causing Radio Frequency Interference (RFI), ideally (though not a necessity) with the cable being put through the clamp twice to create a loop.

This is the size that best fits RG58 and similar cables.

Dimensions are roughly 35mm long x 20mm diameter with the internal diameter of the ferrite being 9mm