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/B09927Q75J...D20H0110T03KWC
https://www.amazon.com/dp/B08FB2XDXF...RQ72R18T4JC89W
I have what I believe is that bottom 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. The roughly 2mm width of the wrenches linked allows you to get on the nut easily. Anything thicker than that and life will suck.