I went and bought one of these allen keys to disassemble a well worn piece of Ikea furniture, only to unzip the back of the chair during disassembly, to find the allen wrench.

Spent a good half hour looking for the manual online, only to remember Ikea manuals are all pictures. I found the manual and was reminded of this. About to give up, I took a risk, and thankfully bought the right sized key. Allen wrenches are made in both Imperial and Metric scales; Ikea, being Swedish, meant correctly picking metric.

I later looked through the manual, and on the 2nd to last page, it documented the correct key size.