We list maximum tire width recommendation for all our models. Find the frame you are wondering about here: http://surlybikes.com/bikes/
click, on frame highlights and then scroll towards the bottom of this page for that information. The max tire size on a Surly is generally larger than most bikes. This is because we love big tires - they mellow out life's harsh bits and they help increase the life of your frame, wheels, and components with their awesome passive suspension qualities.
That said, we have not tried every tire on every frame. Some manufacturers' tires are a bit smaller than they claim, some are a bit larger, some measure overall tire width, others measure just the casing and don't factor in the extra width knobs add. As well, max tire size in the frame can vary depending on where in a horizontal or semi-horizontal dropout your wheel sits (run your wheel all the way forward with the biggest tire we list, and the tire may rub the stays. If we say a frame will fit up to a 45mm tire, that does not mean that EVERY 45mm tire will fit in any gear combo. So take some personal responsibility and use the absolute largest tire with some informed caution.