5.5mm, and I strongly suggest getting a thin midget wrench instead of going with the typical bicycle style wrench you usually find in that size.
https://www.amazon.com/dp/B08FB2XDXF...RQ72R18T4JC89W
I have that set marked with the Gear Wrench brand name and having done installs with the 5.5mm wrench in that set vs. the typical option, there is absolutely no comparison. Buy the midget wrench set and make your life easier.
Note that any old 5.5mm wrench won't cut it, as standard wrenches are too thick to fit under the front sight base to get at the locking nut holding the sight on.