I'm assuming you mean crank length when your refer to "Stroke".
A shorter crank is easier to "spin", good for sprinting, a longer crank gives you more leverage for climbing or time trialing. Track Sprinters use 165 to 170cm cranks, Time Trialists tend to use longer 172.5 to 180cm cranks. Short people tend to use short cranks 165cm, tall people tend to use long cranks 175-180 cm, riders with long femurs tend to use longer cranks as well, (LeMond 5' 9" used 172.5s-175s for time trials, Indurain 6' 3" used 180s?-185s for time trials). Most mountian bikes come with 175cm cranks. If you have bad knees, or want to avoid them, choose the shorter options.
If your're over six foot a 172.5 will probably work fine
If you're under 5' 3" or so you might consider a 165.
There are NO FORMULAS or strict guidelines to determine crank length, that I know of, basically you need to GUESS.
Personally, I'm 5' 9" or 5' 81/2", depending on which leg I'm standing on, and use 170 cm cranks. I have bad knees and cannot use 175cm cranks. I have a bike with 172.5 cranks that I use for climbing or time trialing, but I have to be careful which gears I select, or my knees will hurt.