Yea basically your head chooses it for you...I like the X-11s but nothing fits better than the Arai...in fact, picking up my new Nakano 2 Green Rx7 this week! Wooo!
I've never had the problem you are talking about with my Quantum F. Been a damn good helmet actually, and the most comfortable thing I've ever put on. Shoei's get me right on the top of the head, hell I can barely get a RF1000 on without jumping up a size or 2. Shoei's tend to offer a bit more frontal area too, in case that is a consideration for you (ex: you have a huge nose :P ).
When choosing a size, make sure to focus on the top of the head and get one that is tight yet not pressing on your temples or hairline. The cheek pads on these things break in quite a good bit. If I put a new Large Quantum on then put my old one on, it feels like a whole size difference.
All the Arai's now offer fully removable liners (Signet GTR,Quan2,AastralX,RX7). I know the Shoei X11 does, the RF1000 only the cheekpads come out...not to sure about the Z-Two.
As far as Venmar, nice looking helmets. I tried on the carbon fiber one a month ago in an XL (I normally wear a Large). My only beef is on the back of the helmet. It has this hard foam piece that digs in where your spine meets your head....well...my head anyways. Not the helmet for me, though I still have freaky dreams over that CF helmet ;) .
2005 RSX Type S Silver - Still in denial that I own a Honda
2004 ZX-10R Black - Sold :(
1968 Mustang Black 302 - Awaiting resurrection