Although you can make an arm of any length to mount to a motor shaft using these two things:
http://www.thingiverse.com/thing:6452
http://www.thingiverse.com/thing:6354

and some suitable rod/tube stock, sometimes I just want a quick and easy arm of a given length that is totally printable.

This thing is kind of a mash up of the other two things. On the one end, you can mount it to your 5mm motor shaft (the shoulder). On the other, it has a ball bearing to form a 1DOF joint (the elbow). Between the two is a printed cylinder that forms the 'bone' of the arm.

You can see in the picture how to use the thing, or at least one usage of it.

Of course it's all parameterized, so you can create whatever size you like, limited by the capabilities of your printer. The default size has a distance of 100mm on centers, fits a motor shaft of 5mm, and accomodates a 608 bearing.

1) Dream up a robot that needs some 1 DOF (Degree Of Freedom) segments
2) Print a few of these, fit to size
3) Fit a Pololu universal shaft mount into the side with all the holes
4) Press fit a 608 bearing into the other side
5) Give your robot some bones
6) Rejoice!

You can change many things fairly easily in the OpenScad file, so print to suit your needs.

On my Up! printer, I printed diagonally to make it fit comfortably on the 144mm platform. On something like a Mendel, you can print much larger, only limited by the amount of torque whatever you're attaching it to can ultimately handle.

At this size, and with a joint and carbon fiber lower arm, my nema 17 motor from Adafruit can easily whip it around without stalling.