following these instructions link
i'm 23 1/8" head size

I needed to order some spare helmets, so my wife and I tried a bunch of helmets today after reading that helmets are a personal fits. For us, this statement was very true.

They only had one bell helmet, a solar. both of us found it to tight on the fore and aft of our heads. I guess our heads are too oblang shaped for this bell.. We found the Giros to fit much better.

Then tried the giro skyla (women specific) - way small for both of us

All the giro universal fit helmets fit the same for us. They had 3-4 lines of helmets starting with the indicator (some of them were last year's model I believe) The costlier lines had a little more backside protection and various vent patterns. There also were variation in the internal padding pattern, but it didn't mean any real difference for us. Since the indicator was the cheapest and lightest of the selection (less backside protection), we both chose this one. Over time I find lightness in helmets to be a comfort factor as well.

In $60+ range, the giros came in sizes: S M L.
I was assuming these would fit better than "universal fit", but was mistaken. I tried the phase and another whose name slips my mind. The M was too small while the L was too large. Large could fit tightened up, but not as naturally as the Universal fit giros. the L also looked uglier since it was even bulkier and larger than the Uni fit giros. I'm guess Uni fits must be a medium-large in size which was perfect for us.

I chose white helmets for heat reflection reasons.