For whatever reason conversion expands the file size about 3.5 times. So 26 Mb of FBX motion would result in about 90 Mb of .rlMotion file.
However if you later do a "collect clip" and "Add Motion to Library" as rlMotion it would probably come down to about 30 MB and even smaller for iMotion.

Something RL should work on for optimization.

Of course, the file can be split into two clips, and in that case, it works perfectly. However, I’m reviewing iClone specifically to handle larger files in one go. Personally, I don’t consider 26MB to be particularly large.

FYI, the 26MB figure comes from my testing when I split the file and found the point at which it starts to work properly.

eatsky I don't know xsens, as I'm only using Axis Neuron. Do you have an option that allows to export the motion clip in 2 clips, each one being the approximate half ?

Thanks for the suggestion.
I confirmed that when I converted the animation FBX, it was successfully saved without crashing in the C:\Users\Public\Documents\Reallusion\Reallusion Custom\Animation\Motion\External Motion folder.
However, as soon as I assign the saved .rlMotion file to the motion dummy, iClone crashes immediately.

Try different approach.
Instead of drag/drop, do not select any character and use Import > Convert External Motion from the menu.
It would convert and create a motion in the designated folder only.
If it does not crash, try to apply created motion to a character then.

I'm using MVN Animator Pro from Xsens, so when I import the data, I select the 'xsens' option.

what mocap software are you using ?

Thank you for your suggestion.
I tried it, but the issue remains the same.
When I drag and drop the large animation FBX into an empty space in iClone, it works fine.
However, if I drag and drop the same FBX onto the motion dummy in iClone, the program crashes.
Without the motion dummy, it seems I can't utilize the IK mode in Edit Motion Layer.

Please try increasing the frame-count of the timeline in the Project panel.

I had a similar issue  but with Perception Neuron. Never had a problem before, only this tipe the file was more than 30mb. Iclone crashed everytime. Only solution I found waa to export the raw animation from Axis Neuron in 2 clips, cut in half. It worked this way...