Background: Computed tomography (CT) is currently one of the most useful diagnostic imaging tools for evaluating cardiac disorders in humans and animals. However, studies concerning CT and the feline heart remain scarce. Aim: To create measuring techniques for the feline heart size on CT and to determine the relationships of feline heart size observed on CT with internal parameters including age, bodyweight (BW), and sex. Methods: Four parameters observed on CT including thoracic height/width ratio (THW), relative heart area (RHA), CT vertebral heart score (ctVHS), transverse vertebral heart score (tVHS) in 1.25 mm slice thickness, and both pre- and post-contrast enhanced images were examined. Additionally, radiographic vertebral heart score (rVHS) was also evaluated. Results: THW was significantly influenced by age (p < 0.05). RHA was affected by age and gonadal status of cats (p = 0.001 and p = 0.016, respectively). Age also significantly affected tVHS (p = 0.038). Interestingly, ctVHS was not influenced by age, sex, gonadal status, or BW. tVHS and ctVHS had significant moderately positive correlations with rVHS (r = 0.476; p = 0.048 and r = 0.6112; p = 0.011, respectively). THW and RHA had non-significant correlations with rVHS (r = 0.2642; p = 0.302 and r = 0.1920; p = 0.455, respectively). Conclusion: CT heart size evaluation can be performed in both pre- and post-contrast enhanced images of 1.25 mm slice thickness. tVHS and ctVHS are recommended parameters for evaluating feline heart size in clinical practice.