Solution
LUT Size is not the same as LUT Step Size.
A lookup table (LUT) contains equally spaced PA input signal power levels and their corresponding correction factors. The separation between these levels is the LUT Step Size, and the total number of levels is the LUT Size. For a given input signal power range, you only need to specify one of these (LUT Size or LUT Step Size) to define the power levels in the LUT.
RFmx allows configuring LUT Step Size, but not the LUT Size.
There is no fixed default value of LUT Size, since it is indirectly computed by LUT Step Size (default = 0.1dB) and input signal power range.
Assuming that the LUT Type is "Log", and the Threshold Type is "Relative" (both being their respective default values), the size of the LUT array can be inferred from the
Threshold (dB) and
LUT Step Size properties:
LUTSize = (RelativeThreshold / LUTStepSize) + SomeConstant
where SomeConstant refers to any additional LUT entires we may add around the beginning or end of the LUT.