The external descriptions of a character refer to their features that can be readily observed. It is basically what they look like. This can include their age, facial features, clothing, hairstyle, and anything else that can be seen by others. This is different from their internal features, such as their emotional state, values, and desires.
In The Crucible, Arthur Miller provides some of the external descriptions of many of the characters, usually through exposition and stage notes. Many of these can be found in the early scenes of the play, particularly when the characters are first introduced. Miller also provides descriptions in the character list.
Even though you do not have a copy of the book, it may be possible to find the text online for free. If that fails, the book is likely also available at your local library.
By looking through these descriptions of John Proctor, you will see him described as "powerful of body" and in his "middle thirties." This means that he is a muscular man, likely taller than the other characters, and in good physical health.
Abigail Williams is a young woman, an adolescent really. She is seventeen years old and described as "strikingly beautiful."
This being a play, many of the external descriptions of the characters are kept relatively vague. This is done so that casting directors have more leeway in who they cast in these roles. General physical descriptions are sometimes given, but details, such as hair and eye color, are not included so that more actors have the chance to fit the role.