Skullo 

How does one determine stem length? Is it measured from center to center?

seely

Yes, center of the clamp to the center of the steerer tube.

Retro Grouch 

Well, there are a couple of different measurements in use.

I'd say that "extension" is by far the most common. It's the center-to-center measurement from the steerer tube clamp to the handlebar clamp along the length of the stem.

"Reach" is a little different. It's still a center-to-center measurement but it's on an imaginary line parallel with the ground.

Skullo 

I bought a new (to me) road bike ) Fuji Roubaix Pro with a ritchey stem that is 100mm center to center. I want to replace with ritchey adjustable stem, They come in 80, 100 and 120 mm. I gave up the road bike a few years ago because of wrist and hand pain due to carpal tunnel. Should I stay with the 100 or make a change in stem length.I am not looking to put myself in an upright position as I realize that lower is better on a road bike. I got a great deal on this bike with 200 miles on it and really want it to work out. I have been riding a custom mountain bike that fits well and able to do long rides and centurys, but this geometry it way different in comparison.

waterrockets 

You should research bike fit as much as you can. Consider paying for a professional fit at a local shop.

Road bikes should be much more comfortable for your wrists than mountian bikes because you have so many hand positions available. The caveat is that if you're not producing enough power continuously, you will be bearing a lot more weight on your hands on a road bike. One trick some do to mitigate this is to leave the hoods further up on the bars so your "cruising" position is a little higher, then your drops still get you nice and low. (I tend not to like this setup because of brake lever reach in the drops but it's a really popular setup anyway)

onbike 1939

snip<One trick some do to mitigate this is to leave the hoods further up on the bars so your "cruising" position is a little higher, then your drops still get you nice and low. (I tend not to like this setup because of brake lever reach in the drops but it's a really popular setup anyway)>

I agree it can be difficult re reaching the brakes but there is a way.
Rotate the bars so that when sighting along the bottom flat, it is aligned with the rear dropouts. Bring up the brake levers so that they follow the level of the top of the bars. In this position the brakes should be easily reached using two or three fingers from the drops position. If you know this already, my apologies.

I_bRAD

Parallel to the ground, or normal to the head tube?

waterrockets 

Well, the reach problem is related to the angle of approach for your forearm. With your wrists straight, elbows bent as normal, a pointed index finger should land up on the lever somewhere. Rotating the bars up isn't going to change the angle of your forearm and wrist when riding in the drops.

onbike 1939

That may be true, but I was commenting on finding a position which allows you to ride comfortably on the hoods and the drops. I would have thought that both top tube length and bar height would have a bearing on reach.

waterrockets 

Sorry for the confusion, but I'm talking about reaching the brakes, not top-tube + stem reach! If you raise the hoods relative to the line your forearm draws past the bars, they are tougher to reach. You have to bend your wrist up to reach them. Your index and middle fingers should go straight to the lever if you extend them without bending your wrist.

onbike 1939

+1

Retro Grouch 

Parallel to the ground. If you have a threadless stem that has a lot of rise angle it might have a lot of extension but very little reach. You have to be careful when ordering stems with big rise angles because, as you raise the handlebars, the handlebars keep coming closer to you on a horizontal line.