Concerning RtMidi: They apparently have a (statically defined) maximum length for SysEx messages (1024 bytes), which I find potentially dangerous and questionable - OpenMPT itself cannot currently make use of such long messages, but VST could.

So, RtMidi uses:

#define  RT_SYSEX_BUFFER_SIZE 1024
#define  RT_SYSEX_BUFFER_COUNT 4

and PortMidi uses (portmidi.c and pmwinmm.c, we enter that code with bufferSize==0):

if (bufferSize <= 0) bufferSize = 256; /* default buffer size */
...
midi->buffer_len = bufferSize; /* portMidi input storage */
...
#define INPUT_SYSEX_LEN 64
#define MIN_INPUT_BUFFERS 16
...
int max_sysex_len = midi->buffer_len * 4;
int num_input_buffers = max_sysex_len / INPUT_SYSEX_LEN;
...
if (num_input_buffers < MIN_INPUT_BUFFERS)
    num_input_buffers = MIN_INPUT_BUFFERS;

So, yes, the constants and overall buffering strategy is a little different, but not fundamentally so.

AFAIK a fixed limit (which has to be set upfront) is kind of imposed by winmm anyway - but I have not done any WINAPI MIDI in years, let alone sysex handling, so I might be wrong here.

Closing this as we got rid of PortMidi entirely, so there is nothing to fix anymore.