How to setting automatic change data on form

1. How i can show string "size" in form "width x Length x Height CM"

2. Auto change data when i edit on string "width" on instantly.

eg. width = 73

      Size = 73X6X5cm

3. Can add data to size

eg. Size = 73X6X5cm (9ea)

4. These value update to table

5. Ax 4.0