I expect when you say length you mean height...
You should know that .
If you know the ratio of height to diameter/radius, you'll be able to substitute it into the volume equation and solve for one of the variables.
Okay, so basically what I'm trying to figure out is how to find the dimensions (length and diameter) of a cylinder when I know the volume and the ratio of length to diameter (for instance, a cylinder x times longer than it is wide). Unfortunately, I haven't been able to figure out how to do so on my own and google hasn't exactly been helpful. I'm quite stumped, so I would greatly appreciate any help.