Personal breathalyzers come in a few basic sizes: hand-held, pocket-sized and keychain.
Hand-held devices are the largest portable breathalyzers, tend to be more expensive, cannot fit easily into a pocket or purse and require carrying mouthpieces. But they can be more accurate, and because they use mouthpieces, they can be easily shared among individuals.
Pocket breathalyzers are more affordable and do not typically use mouthpieces; you blow across them without making mouth contact. So they can be shared among individuals, but they are also not very accurate because they cannot capture a deep-lung breath sample.
Keychain devices are the smallest breathalyzers and can be easily carried with you wherever you go. Most keychain devices do not require mouth contact, so you do not have to carry mouthpieces with you, but most are less accurate because they do not capture a deep-lung breath sample. Some keychain breathalyzers require mouth contact and will thus get a better breath sample and give a more accurate reading, but this type is not meant to be shared.