A quick definition before we continue, just so we're all singing from the same sheet - because there's some confusion about the difference between a vibrator and a dildo.
A lot of people think the words 'vibrator' and 'dildo' are interchangeable, but they're not. To qualify as a dildo, a sex toy must be designed specifically for penetration. A dildo doesn't necessarily vibrate (though some do), whereas a vibrator MUST vibrate and can be any shape. In short, if it's phallic in nature, it's a dildo. On the other hand, if a sex toy vibrates but is not phallic, then it's a vibrator, not a dildo.
For the sake of this buyer's guide, when we say 'dildo,' we're referring specifically to non-vibrating, phallic sex toys.
This is pretty straightforward: if you're relatively new to sex toys - and dildos in particular - then stick to the narrower ones. The narrower your dildo is, the more comfortable it is. The more comfortable it is, the more you're likely to use it and, ultimately, the more pleasure you'll get from it.
If you're a bit more experienced, or you crave a more challenging, fulfilling feeling, then by all means choose a wider, longer sex toy. In fact, we recommend a double-ended dildo, so you can use both ends simultaneously to recreate the sensation of double-penetration.
TOP TIP: girth is MUCH more important than length, when it comes to choosing a dildo. A dildo can be three feet long, but that doesn't mean you have to use all three feet of it - so there's no such thing as 'too long'. But there is such a thing as 'too thick.'
Your choice of dildo will also rely heavily on the way in which you intend to use it. If you're using it alone, then your choice is an awful lot freer, and you can afford to be as selfish as you like. Which is great. But if you're using it with a partner, then we recommend a double ended dildo. And if you're using a dildo primarily for anal masturbation, then we suggest you choose a relatively narrow toy with either a flared base or with a very long handle.