I just bought a new TLD A2 helmet with the MIPS system and it was far from the brand, helmet or price point I had initially aimed for. Compared to how older style helmets fit, the MIPS system just feels weird, at least at first, and I was surprised at how many helmets I tried on that just didn’t feel right – I must’ve tried on about 25 different brands, models and sizes. I also found myself trying on some Small sizes whereas older Small bike helmets would never have fit, so I don’t know if MIPS helmets are always consistently a tighter fit. The helmet I eventually got is a MD/LG, which is the size I was wearing before with my old Bell helmet. It might be more of a brand/model/design thing.
I don’t know anything about how the 661 EVO AM Patrol would fit for you and my recent experience tells me the only way you’ll know is when you try it on. So my advice based on my limited experience is if you’re going to mail order a helmet without trying it on first, first make sure the return policy is easy and efficient.
In most (all?) MIPS helmets, the MIPS liner is separate from the retention band. So unless you have a really “tall” head, the MIPS liner shouldn’t affect the circumferential fit. Basically you’re measuring the diameter of your head at the widest point (59cm), and the helmet size refers to the max/min circumference of the retention strap.
If it were me, and I were right on the line, I would go with the M/L. When a helmet is too big, you have to really cinch down the retention band to keep the helmet from bobbing, which can be headache inducing. 🙂