Trouble is finding a helmet isn't just the right size, it is also fit. I was lucky in our local trials supplier (plug, Hard Rock Trials Supply) had a decent assortment of helmets in stock. It was amazing the number of different fits there were from different manufacturers. Some fit like I put a salad bowl on my head.It was on for a second and was off just as fast.
That is where the flaw in internet mail order comes in. You see pictures, read reviews. But when it comes to something that has to interface with your body you really need to feel it first hand. You cannot fit a helmet by size alone.
So keep that in mind when you read reviews of a said helmet.
Recently I needed to add a knee brace to my gear collection. The shops in town with the best selections of gear had one make, model and size on the shelf. But they could order anything I wanted, so long as I paid for it first. But they would give me my money back if I didn't want it. I knew of a store about 5 hours away that had stock. Next time I did a road trip that way I stopped in and tried on all of the knee braces. I did at least a half dozen different models. They all "fit" but only 2 "fit right". How I miss good brick and mortar stores with selection. I know it is nearly impossible to keep up with the internet as people will go to lengths just to save a buck, the online selection is so much larger, but the personalized attention is gone.
So with whatever helmet you get, make sure it not only fits, but fits right. I have a street helmet bought off the internet that fits, and fits OK, but after about an hour it gives me headaches. Good thing work is only 15 minutes away. But it does keep me from taking longer rides. I really should get something that is more comfortable, but I am giving up on most of my street riding to enjoy trials instead.