Kabuto

Officially, the maximum tire clearance is 33 mm. Unofficially, I suppose you could go a few mm wider? Is anyone running wider tires on a Roubaix?

I'm thinking of trying Challenge Strada Bianca Pro HTLR 36 mm tubeless tires, but maybe that's stretching it a bit too far?

PeteHski

I'm afraid I can't answer your question, but it will also depend on your internal rim width. For example, a 36C tyre might just clear when fitted on a narrow rim, but rub when fitted on a wider rim. Just something to bear in mind.

Kabuto

Yes, good point.

The rims I plan to use have an internal width of 20.9 mm, which is I suppose about standard this days, maybe even a little on the small side.

PeteHski

Some tyre manufacturers quote actual tyre width for various different rim widths (Pirelli do this with some tyres), which might help you out. I would at least try to find out what rim width their nominal tyre width is stated for. I don't think all tyre manufacturers are on the same page in that respect. So a 36C Challenge tyre could measure wide/narrow compared to a 36C tyre from another brand.

Iride01 

If you just look at your bike with the current wheels and tires on it you should be able to tell how much bigger you can go.

Just find the places around the tire where the clearance is the smallest amount. Look for both sidewall clearance and tread clearance along the greatest circumference of the tire. That's how much bigger your tire can get. Usually for the rear the seat tube will be the issue. But possibly the chain stays or if rim brakes the center part of the calipers where they mount to the frame. For front the front the fork crown or again if rim brake then the caliper mount point.

Tires grow in diameter by about the same amount as width is increased. But if you get a tire with more aggressive tread pattern, it might be more.