Solution
The maximum waveform size is determined by the number of samples rather than the file size in bytes. The PXIe-5673E uses a 16-bit digital-to-analog converter (DAC) and stores both I (in-phase) and Q (quadrature) data for each RF sample. This means each RF sample requires 32 bits (4 bytes). Therefore, the theoretical maximum is the instrument's memory size divided by the 4 bytes/sample constant. For a model with 512 MB of onboard memory, this means a the uploaded waveform can have a total of 128 MSamples.
In practice, the instrument may have slightly more or less memory than specified. For example, a given unit with 512 MB of memory actually had a total of 536 MB of usable memory, and can store approximately 134 MSamples.
Finally, the PXIe-5673E can store multiple waveforms simultaneously, but the combined sample count of all waveforms must fit within the available memory.