An ultrasound system for planning a surgical implantation of an implantable device produces two or three dimensional ultrasound images of the site of the surgical implantation. An image of a sizer for an implantable device comprises a virtual sizer which is scaled to the scale of the anatomy in the ultrasound image. A user manipulates the virtual sizer on the display in relation to the anatomy in the ultrasound image to ascertain whether the virtual sizer, and hence its corresponding implantable device, fits the patient's anatomy. In place of the anatomical ultrasound image a model of the anatomy can be produced from the ultrasound image data and used for sizing. When sizing is done with 3D ultrasound images, the fit of the virtual sizer and the anatomy can be studied by rotating and tilting the two images together. Imprecision in the fit of the virtual sizer and anatomy can be display with highlighting.