The invention relates to a spatial dimension determination device for determining a spatial dimension of a target element like a vessel within an object (9), for instance, within a person. Movement parameters describing a movement of an introduction element (4) moved by a moving unit (2) like a robot and several images showing the introduction element are used for determining a dimensional relation between an image dimension and a real dimension. A further image showing the target element is provided, wherein a real dimension of the target element is determined based on an image dimension of the target element in the further image and based on the determined dimensional relation. An unknown real physical dimension of a target element within the object can therefore accurately be determined from an image of the target element by using, inter alia, the movement parameters.