Watch the Left-Hand Techniques 2 online guitar lesson by Bruce Arnold from Guitar Physiology Survival Guide
In general you should keep your fingers relaxed and not spread them out unless you absolutely have to. This is usually the hardest thing for students to master. Remember you want to keep your hand close to the position it is in when you are just relaxing it beside your body.
The movement from one note to another will come from your elbow. Think of your hand as a claw that has to slide around to get to notes rather than stretching your fingers out and reaching for notes. Yes, the size of your hand will greatly affect how much your fingers tense up. If you have huge hands or long fingers you will have fewer problems, but if your hands are small or you have short fingers the application of this concept will be crucial. There are of course certain times that you will spread you fingers out but this should be the exception, not the rule.