Exact size is partly a function of the user. It's ironic that warnings on prescription bottles are often small even though the elderly, who have reduced vision, use prescription drugs with high frequency. Moreover, people often perceive the wrong part of the warning. There are many cases, for example, of a nurse looking at a vial and then injecting the patient with the wrong drug. In most cases, the nurse noticed the label but failed to perceive it.
People have a tendency to generalize the cues that they use to make decisions.
Once the warning is perceived, the user must properly understand its meaning. The message should be clear and easily understood, but it is difficult to ensure clarity for several reasons. Sometimes the message is simply too vague. Users may also fail to understand the warning if they have little familiarity with the product and it operation. The words may be comprehended but the relationship to produce use unclear.