I have a Trek Silque, 2017. What centimeter size should I be using in my computer for figuring correct distance?
Very roughly, you take the ISO size (on the side of the tire, in a form like "38-584"), add the two times the first number to the second number (eg, 76 + 584), and that gives you the wheel diameter in mm.
(There is the problem that the tire width value is far from exact, but it's about the best you can go on.)
If the application you're dealing with wants the circumference then you multiply the above result by pi (3.14).
From a web search I see that the original tires on the bike were likely 700x25c. The "700" rim has an ISO diameter of 622. The 25 width is probably close enough to the ISO value. So 622 + 50 = 672 mm diameter, or 2110 mm circumference. (Of course, divide the numbers by 10 for cm values.)
Or, of course, you can simply walk the bike along the floor for 5 or 10 complete wheel revolutions, measure the distance traveled, and calculate from that.