All Fords use Bosch type injectors. The injectors are the same for both the newer non-return fuel systems and the older return-style. The following Ford injector sizes will work in your 4cyl Zetec Contour: 19lb, 24lb, 30lb, and 36lb. The use of 42lb or larger injectors requires a low impedence injector driver in your EEC. However, your MAF meter MUST be recalibrated for the appropriate injector size. It is important for you to know that bigger injectors do not make more power.
The only time you really need to increase your injector size is if you are maxing out the flow of your current injectors. Normally this only happens when you've made SUBSTANTIAL airflow improvements. The two ways an injector performs are pulse width and duty cycle. These are terms for the time an injector stays open and the amount of fuel it delivers. Typically, an injector should not exceed 90% duty cycle. If so, you may need to step up injector size. On the '98 and older return-style fuel systems, the easier (cheaper) route is to increase fuel pressure a bit. In Ford cars, the designers usually over-engineer some systems for relaibility purposes. The chances are, even after bolting on a few simple mods like a hi-flow intake pipe and better exhaust, your stock injectors will be able to supply the extra fuel needed just fine. You can find out your current injector duty cycle by hooking up an OBD-II scan tool. Some shops that do dyno-tuning have equipment that can record and play back EEC readings in real time. The more expensive scan tools display both pulse width and duty cycle.
An example of proper injector sizing is my '95 Mustang 5.0. It came with 19lb injectors. My first mods were: cold air pipe, throttle body, full exhaust (headers, H-pipe, muffers). The stock injectors worked fine with the extra air flow. My next set of mods were more drastic: larger port aluminum heads and better intake manifold. With these mods, my stock injectors were just over 95% duty cycle. I then switched to 24lb injectors. The bigger 24lb injector size gave me more "headroom" and less chance of a damaging lean condition than the maxed out stock 19lb. When I installed my Vortech, I could have gone with the next-up 30lb size, but I was advised that they would be at their limit with the blower and desired boost level. The current 36lb injectors leave me some room to play with if I want to step up the boost. The down-side of going with an injector that's too big for your combination, is poor drivability and fuel loading up the cylinders at idle. Unless you're adding a blower or dry nitrous kit, your stock injectors should be fine.
