I'm trying to size the rafters I need to use to build a roof over an existing sun room. The span is 21 feet from wall to support beams and I'm thinking of using 24' - 2 x 8 for the rafters. The intent is to rarely if ever go out on the roof once it's up, it's merely to insulate the roof of the sun room. It sounds like hail hitting the roof with the softest rain now.

Should we size up to 2 x 10 or will the 2 x 8 with decking, tar paper and 3 tab shingles be enough?

A lot depends on what part of the world you live in. Your span of 21' is more than most building codes will allow without some support in the middle. Also... composite shingles are usually considered a "heavy weight" roof and will decrease the span allowances. Other factors that come into play are:

1. the slope of the new roof
2. the type of roof you plan to install over the rafters
3. any wind or snow loads that your area is subject to.

If you contact your local building department you should be able to get a standard "hand out" that describes the codes relative to footing size, header spans and rafter spans.

Good luck.

its to long a span for 2x8s. Without some type of bracing, just the span will cause them to sagg,

we just built a sunroom with a 34' shed roof, and we used 2x6, but, they were braced quite often, there were actually 2 slops on this shed roof, the longest was i think 20ft, span, you can span what your talking about but you'll have to have some type of beam spaning the oppositte way to build a knee wall for support,
snow and ice will play a big factor also in how it will hold up. Code for most is a minimum of a 4 and 12, but you really need more than that, but that will depend on what your tieing into as to what you can get away with, good luck