i have a small Duke and it fits under the seat with a bit of pressure on it. Not sure if the small has the same size shell as the medium or large but i can tell you its a huge helmet shell for a small so i think its the same shell for all with more/less padding depending on size!
Personally the best thing ive done to my pcx is fit the biggest topbox i could get, i can fit my helmet, gloves, boots and trousers all in the box alone. Dont get the Honda box as its overpriced, a 2nd hand or ebay Givi is all you need!
I usually ride with an open-face lid so no problems with fit. However I do own a Caberg 'Justissimo' flip-up (Large) and that fits under my seat snugly. I'm not sure if the Duke is that similar though, sorry.
I have an XL too. It fits in my top box but I haven't even bothered trying to get it under the seat. It is huge! I had to go for one of the more expensive helmets because XL in other types wouldn't fit - that's how big my head is!
Thanks guys. No fighting! :-P I hope to check it today.
Ben - what box did you get? Does it look "a bit too big" as in very practical but looks gigantic? In some pictures the largest boxes dwarf the scooter.
They're not fighting, they're just both twats. If you hang around the forum and read more, you'll see GN2 and you you mucking up threads with their banter everywhere. Never take em seriously.
Back on topic, you might have some trouble -- my small modular helmet BARELY fits. It's an HJC.
Currently ride: Nothing right now - mostly mountain biking with my boys until they're old enough to ride Previously rides: 2011 Honda PCX 125, 2005 V-Strom DL650, 1974 Vespa Ciao, 2011 Honda PCX 170 (tons of mods - takegawa 170cc big bore kit, gears, etc), 1996 Honda Nighthawk 250, 1987 Honda Spree, 2000 KTM 125SX, 2003 Honda Silverwing, 2007 Genuine Buddy 125, 1998 Honda PC800, 2008 Buddy 125 (white), 2008 Buddy 125 (red), 2001 Honda Reflex, 1987 Honda Elite, 1988 Honda Spree, 2007 Yamaha Vino, 2007 Honda Metro, 2x 125cc pure-chinesium dirt bikes
So I tried a Caburg Duke Large in the PCX underseat.
It is possible to close the lid, but you have to put a bit of tension on it to do so. I thought this was a problem as:
It can't be too good to have the catch/hinges under constant pressure.
It can't be to good to have helmet under constant pressure.
If someone sits on your seat while you are away that pressure just increased 100 fold.
So I've picked a Givi E300B (the one suggested by Doble Motorcycles) to go on the White PCX I put a deposit on yesterday! I checked the helmet fitted in the top box with a small amount of rattle room.
gn2 wrote:Depends on the helmet rather than the size.
A large of one helmet model might be bigger than the XL of another.
Mine's an XL and fits easily.
gn2 wrote:Depends on the helmet rather than the size.
A large of one helmet model might be bigger than the XL of another.
Mine's an XL and fits easily.
gn2 wrote:Depends on the helmet rather than the size.
A large of one helmet model might be bigger than the XL of another.
Mine's an XL and fits easily.
Anyone got a Caberg Duke Helmet? Does it fit?
[/URL
If you hang around the forum and read more, you'll see GN2 and you you mucking up threads with their banter everywhere. Never take em seriously. 
