When writing a retirement card for a co-worker, the language should have a positive, encouraging tone, and the message should be focused on gratitude for the person's work or encouragement for the future. Unless she is a close personal friend, the message should be concise.
The specific message is very dependent on your relationship with the co-worker. If the co-worker is someone you do not know well, it is often best to keep it lighthearted or funny. If possible, note a funny or memorable event the two of you shared.
If the co-worker is someone who helped you begin at the company or acted as a mentor, a heartfelt thank you is an acceptable approach. Describe the value that person added to your experience at the company. You can also describe a memorable event with this type of co-worker, but a more sentimental event could have a strong impact here.
Lastly, if the co-worker is someone with whom you have had relational or interpersonal conflicts, a concise note of congratulations is best. Keep your note positive and encouraging. Above all, do not write anything negative, regardless of your feelings towards the person.
The length of these messages depends on whether it is a group card or a personal card. For a group card, a brief sentence with a signature is usually a good length. Do not take up too much space on a group card.