I think the main parameters you would need to look at when deciding on the PLC are number of I/O you require and speed of response desired.
Before getting to that stage I would decide what manufacturer I would want, there are quite a few, but most offer the comparable products, so support in your area maybe an important factor.
Once you've decided the manufacturer, you will find they supply a range to cover all your needs (small, medium and large).
The two most popular, dependng on where you are I would say are Siemens and Allen Bradley, but there are a lot of other choices such as Omron, Mitsibushi, Modicon, etc.