Magnet? Might you be referring to the battery?
It's size 2032 - same as many other car remote fobs. Found easily in countless pharmacies, grocers, big boxes, hardware stores and of course amazon, etc.
*not to scale - b/c I don't know what device you are viewing on
but it's about the size of 2 US quarters stacked
Otherwise there is a 'pill' in the fob's head but it's 1/2 the size of a tic-tac. Without it, you're sunk buying a replacement eBay fob b/c you need to xfer yours over. And if you haven't searched yet regarding replacement keys... you MUST have the barcode that comes w/ them (if a new 'pill' is being introduced and not transferring over an existing one to a new fob) b/c keys [the 'pill'] gets programmed to the car...
Good luck
