Anyone who already has a Bioflow collar will know how robust and durable the strap is. These can be washed in the washing machine or scrubbed in the sink. Just simply slide the magnet off first, although if you forget, you'll know to look on the side of your washing machine drum for the magnet as it will have stuck to it. However as there are no moving parts or batteries etc inside the magnet housing, it will be fine to get it wet.
Something else to be aware of too, if that as this magnet slides, if you find the buckle part of the collar tends to fall to the front of the neck more often than not, then don't be afraid to slide the magnet round nearer to the buckle where it will be nearer to the dogs main veins in the neck.
Long fur around the neck is also something to be aware of - if it is keeping the magnetic field away from the dogs skin, you may want to clip a little of it in that area? Although the bioflow has a powerful depth of field, very thick fur may be reducing the magnetism from reaching the body.
With regards to sizing... if you think your dog could wear a wider strapped collar (there are 2 sizes, small and large) I would always suggest the larger one, based on the fact that you get a bigger magnet, hence more magnetism. If you're unsure, just ask me for help on choosing the right strap.
Again with sizing, the large straps will go very large if needed... Any excess can be cut off and to seal the ends, carefully use a naked flame to gently melt the material. It should only take 3-4 seconds to singe it adequately.
One other thing to consider... anyone who finds that one day they no longer have their dog to put the collar on (I know, very sad but it happens to the best of them) Remember, that magnet will still work against any body... Animal or person and it's been nice to hear how the occasional resourceful customer has taken advantage of this and will admit to using it on various points of pain for themselves or their partners and I've even seen the odd one threaded onto an old watch strap and tucked into the odd sock... If you have one knocking around... Maybe give it a try and don't knock it until you've tried it!