Good size. The AlcoSense Breathalyser looks like a mobile phone (old style). Doesn't always work straight away (flashes up error) although this could be down to the battery level at it works every time when new batteries are inside the machine. When the reading is "low" I would have liked to have seen the reading rather than just "low". I know that means legal but it would still be good to see the reading level. It's a bit pricey but still cheaper than 18 months worth of taxis!