In accordance with the present invention there is provided apparatuses and methods for using a medical instrument including a sizing and positioning adapter. The apparatus comprises an adapter for a medical instrument, the adapt having a body having a shim portion and an instrument holding portion adjacent the shim portion, wherein the shim portion provides an enlarged peripheral surface adjacent the medical instrument such that when the medical instrument is held in the instrument holding portion and the medical instrument and body are inserted through an opening in a patient tissue, at least a portion of the enlarged peripheral surface is in contact with at least a portion of the periphery of the opening in the patient tissue.