Measuring and grading left ventricular (LV) size is essential for diagnostic, treatment, and prognostic purposes. Guidelines for quantifying LV size exist for pediatric and adult patients via M-mode measurements, but no data exist determining how well they agree with one another. The goal of this study was to determine the agreement between pediatric echocardiographic readers (PER), pediatric guidelines, and adult guidelines in assessing LV dilation. A retrospective review of all noncongenital echocardiograms from 9/2002 to 11/2015 that had a left ventricular end-diastolic diameter (LVEDD) >5.8 cm for males and >5.2 cm for females was performed. LV size was graded as normal (Z-score ≤ 2), mild (2 < Z-score ≤ 3), moderate (3 < Z-score ≤ 4), or severe (4 < Z-score) based on pediatric and adult guidelines. PER interpretation was also recorded. Agreement between LV size assessments was determined for these three interpretations. A total of 1489 echocardiograms met the inclusion criteria (654 males:835 females). Males were 19.0 ± 6.9 years old and had a BSA of 1.9 ± 0.3 m², and LVEDD was 6.3 ± 0.5 cm. Females were 18.7 ± 8.3 years old and had a BSA of 1.8 ± 0.3 m², and LVEDD was 5.7 ± 0.5 cm. There was a 63.91% agreement for males and an 81.8% agreement for females between PER and pediatric guidelines in assessing LV size. There was a 39.14% agreement for males and a 14.13% agreement for females between PER and adult guidelines in assessing LV size. There was a 41.44% agreement for males and a 14.49% agreement for females between adult and pediatric guidelines in assessing LV size. These agreement percentages did not change significantly when separating the population into greater than or less than 18 years of age cohorts. Pediatric echocardiographic readers were more consistent in following pediatric guidelines than adult guidelines in assessing LV size. The agreement for PER and pediatric guidelines was poor, especially for females, in relation to adult guidelines when assessing LV size. Further standardization and guidelines are needed for pediatric patients that are adult size.