When calibration tests of dielectric soil water sensors are conducted in a laboratory, it is of great importance to know the appropriate sample size for each sensor used. In the current study, appropriate sample sizes for different dielectric sensors of time domain reflectometry (TDR) systems (TDR100 with CS605 probe and TRIME-EZ), water content reflectometer (WCR) systems (CS615 and CS616), and an ECH2O (EC20) were examined experimentally using air-dried packed soil (Andosol) columns with diameters ranging from 70 to 150 mm; surrounded initially by air and then water. Although the output values of all dielectric sensors increased independently of the sample diameters when the samples were surrounded by water, the significances of these increases depended on the sample diameters. The result showed that a sample diameter adequate for the effective sensor sampling area of the TDR100 with the CS605 was ≥80 mm. Similarly, appropriate sample diameters were ≥150 mm for the TRIME-EZ, ≥100 mm for the CS615 and CS616, and ≥70 mm for the EC20, respectively. The differences in the appropriate sample diameters among sensors are due to the distribution of the electromagnetic energy of probes linked to their design. The results suggest that approximately 70% of the electromagnetic energy or more must be within the sample diameter if the effective sampling area of the probe is to be included within the sample.