There has been many a discussion at all levels of cycling with regards to tyre width and the optimal compromise between drag, grip and comfort. The answer is a little more simple on the whole. Ask yourself a simple question of 'how do I ride'?
If you are racing on the velodrome then you may opt for high pressure tubs of about 20mm. This is due to having no potholes, a banked track and a need for all out speed. As there are no lumps and bumps on a wooden track a higher pressure can be run to reduce drag without effecting comfort. A pressure of about 150psi can be used to get the desired effect, with a narrow tyre used as sufficient grip is given by the banked track.
Compare this to the road rider, who races, then a 23mm tyre is the option most commonly used. A slightly wider tyre that absorbs some of the smaller bumps and does not greatly create drag. These are often a clincher setup so that inner tubes can be changed at the road side, where fitting a new tubular tyre is less practical. Riders who race often will possibly use a tubular tyre but not run the pressures as high as on the track due to the changing road surfaces on their route and for an element of comfort. Also as you are riding outside, the weather can play a part in tyre pressures. A clincher in dry weather is normally 120psi aprox. Where in wet weather this will drop to 90psi aproximately to increase contact to the road.
If you are about to embark on one of the cobbled classics sportive rides then you will need something for comfort. This will normally mean something above a 25mm tyre. The most popular tyre is The Vittoria Open PAVE which comes in a 25mm and a 27mm. If you want something wider again Continental do the Gatorskin in a 32mm!