Width limits on gable dormer??????

The question is when framing a gable dormer where there will not be a supporting wall under the joists (at least not under the entire run), how wide can these be made? Asked a different way, is there a limit to the number of joists that can be removed?
I understand the concept of replacing each removed joist with one on each side when cutting in a dormer.
The entire run is 10 foot from ridge to OS wall. The roof (very old construction), is currently SYP 2x4 24" OC. I realize I will need to use 2x6 to meet current codes. There will be no header. The dormer ridge will run from the existing ridge to the OS wall.
I will check with the local code people. I just don't want to look like a fool when I go in there so I am gathering a little information first.
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