As in the German tank sights? If so, there is a system of measuring that is used and once you LEARN it, you wont have an issue using it.
Read this and learn the "how to" portion. In AH, consider all the tanks close enough the same size for targeting purposes. That write up is for another game, but the German Panzer sight works the same.
http://www.75thguards.com/ww2online/downloads/Zheriz_Ziess_Sight_Guide.pdfThere are other sites out there with more specific detail, but the link above is a good starter.
Dont forget that the Tiger, Panther, and Firefly all have variable zoom. The first power is 2.5X for the Tiger and Panther and the second is a 5X. The Firefly has a 3X and a 6X respectivley. The M4/76mm has a fixed 5X. All else is the lower 1.6X (M3/75mm), 2.4X (Pzr IV), or 2.5X (M4/75, and both T34's).