You could measure the cylinder then try to match it up with teh closest teleflex cylinder. Even if you can't find an exact match, you will be able to figure out the approximate displacement which is what you need for sizing the AP.
For an AP you will need:
1) Steering pump sized to your boat
2) Electric rate compass installed where it won't be affected by other things in the boat
3) GPS, which you probably already have
4) Rudder indicator. Most APs will work without a rudder position sensor, but I'd recommend you still install one. Especially on a larger boat, you will get better performance.
5) AP computer. This is the brains of the system
6) One or more control heads. These are the displays by which you control the AP.
I think this arrangement of components is the same for all the major manufacturers.
If properly installed, and assuming your chart plotter is relatively current, any modern AP will be able to steer a course (follow a route, or goto a waypoint) expressed by your chart plotter. If your AP and chart plotter are the same brand then you might get a slight added feature where activating a route or a goto will also engage the AP. Some people place high value on this, others place no value of it, especially if it means compromising on the underlying AP selection.
Simrad makes an excellent AP - arguably the best - but I've heard the other manufacturer's a pretty good too. I would only buy a non-Simrad AP if I felt the auto-engage-from-the-chart-plotter was a must-have feature, and even then, be certain that your particular system does that.
All the manufacturers have their installation manuals available on-line. I'd strongly suggest downloading and reading one or more of them. They will answer your questions way better that I can. That's how I learned about it