A wound therapy system includes a therapy unit, a wound dressing and an optional controller. The therapy unit is configured to deliver instillation fluid to a wound site. The wound dressing is formed from a plurality of discrete, individual blocks that are selectively separable from one another along a plurality of separation-lines, allowing the wound dressing to be customized to the shape and size of the wound site. By calculating the remaining blocks that define the wound dressing following customization, the volume of the wound dressing may be determined. The controller may be configured to deliver fluid to the wound site based on this calculated volume. The controller may also optionally be used to gauge the healing of the wound site over time by monitoring the changes in volume of customized wound dressings as wound dressings are replaced during the course of treatment of the wound site.