i have a wee head (about 21.25" and slightly oval - usually size S in any bicycle/motorcycle helmet), but the size medium giro advantage II fit me, so their small would be more like an extra small.
my S/M rudy syton is *just* small enough (with it adjusted as tight as it will go), but isn't as snug as i'd prefer - i have to keep pushing it back on my head as it will droop forward a bit.
my bell mtb helmet in size small fits perfectly, and can adjust smaller than i need, but not much bigger - makes it tough to get an earband under it sometimes.
the last helmet i bought is a louis garneau road helmet in size small, and i was very pleased that the "spiderlock elite" retention adjustment system actually had room to go both smaller (by quite a bit) and larger than i needed. maybe the rocket? or, if you want a smaller shell, perhaps the chrono?
cheers!
-mistress k
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ill advised racing inc.