Systems and methods for assisting the placement of a catheter within a vessel of a patient via an ultrasound imaging system are disclosed. The systems and methods described herein relate to an automatic size comparison tool to enable a clinician to determine, prior to insertion of the catheter, whether a particularly sized catheter will fit within a designated vessel of the patient without violating a user-defined rule setting a maximum percentage of the vessel that may be occupied by the catheter. This in turn ensures that the catheter is properly sized for the vessel in which it is placed, according to user-defined preferences. Though described herein as implemented in connection with an ultrasound imaging system, in other embodiments the system and methods described herein can be included with other devices as well.