tube and fender size for Surly cross check

[Sky King]

I asked the Bike Hermit about this. Here is his reply:

For a 700x 32 tire, the second number is the nominal width in millimeters of the inflated tire. In practice a 32 width can be anywhere from 28mm to 33mm or so. Tubes are balloons and are sized to fit a range of widths...just get tubes that include the width of your tire within the range. The other number (32mm PRESTA) is the length of the valve stem.
The Surly website means tires up to 45mm wide will fit, but if you want to use fenders too, go with a narrower tire because the fenders need to be wider than the tire. I use 700x 35 Panaracer Pasela tires and 700x 45 Zefal fenders on my Cross Check. In your example the 700 x 45 fenders will be the best choice. I know that fenders up to 50mm wide will fit on these bikes. More on fenders at Bike Touring News

[Velocivixen]

Fenders need to be 10-12 mm wider than the tires. So if your tires are, for example, 700x32c then the fenders should be 42-45mm. I have 700x32c tires and have a 45 mm fender on it with plenty of clearance. The reason that the fenders should be larger is in the event your tire picks up a blob of mud, a rock or whatever it won't get caught between the tire and fender (hopefully). SKS fenders of a secure-release built into the front in case something does get caught up there.