Can I use a 2x8 for a ceiling joist and rafters on a 16' span?
The local IRC book would be handy for you. I have a few screen shots that may help, but only as a guide line. A building inspector will have the final say, but this is what they go by.

This does depend on where you live, for it has to do with snow loads. Roof cover factors in a bit too.

Span does not matter if it is  4/12 pitch or a 12/12 pitch. The span is not measured on the slope.