I have a water tower,and i need to install a valved pipe as shown in the sketch to allow me to take samples of the water.  

How can I size the pipe to get the flow I need from gravity?

Is it as simple as plotting the system curve for the outlet pipe and the flow will be whatever flow corresponds to the driving head (water depth)?

Use a 3/4" pipe.
You will have all the static head from the water level in the tank plus all the static head from the water level in the pipe.  Just stand back a little when you open the valve.  Oh, be sure you use a "globe" valve so you can throttle the flow.

You've drawn a typical new-guy sample line.  If you are in a climate that ever freezes, the dead leg between the tower and the valve will freeze.  It happens every year when winter sneaks up on us.  I address that issue with a cable operated ball valve (just fabricate a piece of lightweight plate in an "L" shape with a hold at the intersection that fits the ball valve stem and put cables in the ends of the "L") located as close as possible to the tower.  Then put a globe valve on the end to throttle the flow to the sample bottle.  After you pull the sample, you can shut the ball valve and re-open the globe valve to drain the line.

You need to think about what else you may want to use this line for in the future.  You may need to use it as a drain, or to inject chemicals, or some other odd thing that you can't pre-engineer.  Consequently, while 3/4" (or 3/8" for that matter) would give your adequate flow for a sample, I usually design this sort of thing as 2-inch (maybe with a 2X3/4" bushing in the end to get a manageable sample size).

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