A sizing catheter and method of measuring a preselected internal opening within a patient to provide a rapid and precise determination of a stretched diameter of the preselected internal opening. The sizing catheter includes a dilation balloon constructed of a thin expandable plastic which is inflatable and is utilized to determine a size of the preselected opening. The dilation balloon is inflated to an inflation threshold, wherein the dilation balloon deforms about the preselected opening and the size of the dilation balloon adjacent the preselected opening approximates a stretched diameter of the preselected opening. The sizing catheter and method may be utilized to determine an appropriate sized occluding device to thereby occlude the preselected opening.