The base dimension is using the value for weight.  

Bug in file Nop.Plugin.Shipping.USPS/USPSComputationMethod.cs in CreateRequest method, near line 110:
var baseusedMeasureDimension = _measureService.GetMeasureWeightById(_measureSettings.BaseDimensionId);

Fix:
var baseusedMeasureDimension = _measureService.GetMeasureDimensionById(_measureSettings.BaseDimensionId);

Paul Rony
SplendidCRM Software, Inc.