This depends mostly on how you intend to use them. If you are just doing occasional work in a home garage, then what you've mentioned would be fine. But if you are going to give it a workout, then upgrade to better quality.
4 1/2 inches for the angle grinder is a very common and useful size. You can use the 4 1/2 inch wheels on the 5 inch grinder you mentioned. 5 inch wheels are not as common although certainly available.
6" bench grinder is the way to go.
By the way, I sell welding and metal working supplies for a living. I'm very familiar with the topic.