Look inside the lens cap - there'll be a "ø" sign followed by a number - like "ø67mm". The number refers to the filter diameter of the lens. If you're in any doubt, take the lens(es) with you when filter shopping.
Assuming the 17-85 is the Canon EF-S 17-85mm f/4-5.6 IS USM, the filter IS a 67mm one (good guess!) as is the 70-200 (if it's the Canon EF 70-200mm f4 L IS USM). Don't be tempted to get the cheapest "protection" filters you can - why risk spoiling the images from expensive lenses by sticking an extra cheap piece of glass in front of it?
FTR, I only use protection filters in conditions where there's a LOT of windblown dust/sand/water - apart from that, I rely on the lens hood and caution to protect the front elements - so far, I have yet to damage a lens.