If possible, you may want to post some of your own drawings where you feel there is trouble with the neck, so we can have an example. It's hard to say what is being done wrong when we can't see it.
On a real person, you can see that the neck is almost as thick as the head.
Of course, this is anime we're talking about, so it's really flexible how thick the neck can be. Many people draw very thin necks. The important thing is to be consistant, and make sure that the neck isn't lopsided.
Experiment and see what width fits your style the best.
I personally find it very useful to study real people and understand their anatomy. Anime is based on humans, after all, and you have to know what a human looks like to draw one.
Some important things to consider when drawing the neck and area around it are 1. The clavicles (or collar bones) those are the bones that go from the top of your arms to the bottom of your neck. 2. You see those lines that go from about the ears to the base of the neck? The ones that form a 'V' shape? Those are made by the sterno-mastoid muscles. A lot of people add those lines for more detail in the neck, and it's important to observe where they start and end.
I hope that was somewhat helpful to you.
heart Please do post some of your pictures so that we can be more specific.