I like what Carleton said.
Even then, high gearing doesn't cause knee issues as much as it REVEALS
Saddle position, cleat position/angle and orthotics can cure any knee problem in a person who can still walk.
As far as online gear calculators go, I prefer Bare Knuckle Brigade's Rabbit applet:
A high gear ratio allows a new fixed gear rider, with a brake, to ride very fast in the flat lands.
In comparison, a low gear ratio allows a new fixed gear rider to develop a better spin, sooner, as well as allowing him better acceleration, better low speed control/balance, and the ability to stop without brakes and without skidding.
When I first started riding fixed, I quickly went UP
in gearing to 82 gear inches (53 X 17), and at this gearing I could fly on flat land and still get up hills.
However, at that high of a gearing, I had trouble getting across intersections from a stop, I became dependent on brakes/skidding/skipping, I had lessened low speed control, and I had less motivation to truly analyze and develop my spin.
Then after about two years of riding fixed I had to go through six months of chemotherapy, and wanting to ride even through chemotherapy, I geared down to 60 gear inches (39 X 17).
To my surprise, my spin quickly improved and my around town times from A to B at 60 gear inches quickly matched my previous times at 82 gear inches.
I completely got rid of my "bounce," and I learned techniques for back pedaling that allowed me to stop as quickly, or more quickly, than I had before with a brake and skidding.
I also found it very easy to accelerate across/through intersections, and I discovered a new ability to ride very slowly and very precisely.
After chemotherapy, I experimented with various gear ratios to see what I could learn, and especially how all these different gear ratios looked/felt from the perspective of good health and tons of energy.
For me, I found that 72 gear inches (47 X 17) represented the highest gear with which I could reliably brake with just back pedaling.
I also found myself gravitating back to 60 gear inches.
I presently have a new bike in the works, and for this bike I have decided to ride at 61 gear inches (45 X 19 with 25mm tires).
I have also realized, for skidding, the rider simply needs to have a prime number cog (17 or 19) in order to maximize the number of skid patches.