People become very attached to their favourite amongst the most-used of all reference books.
However, a few guidelines on selection can easily be established. Let's start with the question of size. The smallest dictionaries, which you really can fit into your pocket, are only suitable for a quick check of spelling and meaning in most commonly-used words. These are mini-books the size of cigarette packets, which often end up in your desk drawer. The Oxford English Minidictionary manages to pack 40,000 entries and 50,000 definitions into a miraculously small space - and throws in a few extra pages which offer solutions to common problems.