Somewhere I have my Headers by Ed design documents that go into details on tube sizes, collector size ratios vs tube size, etc.

What I recall is based on Ed's formula, a 1-7/8" with a 3.0" collector is pretty much right in the middle of his acceptable range; a 3.5" is pretty big for that tube size, unless you're turning a lot of RPM and/or narrow RPM range (stick car vs auto).

With 2" tubes, a 3.5" is also pretty much right in the middle of his acceptable range. I've only seen one prod header that I remember 2" tubes and 3.0" collector, and that was a Hedman Header model which I never considered.

A 3.0" for a 2" tube is definitely on the small side, although in a merge collector it could taper down to 3.0" before expanding back to 3.5". But everything I've heard says you should design that from the start, not just slap a merge collector on a header not designed for one... even though I see that's what a number of header companies do now cuz it's simpler that way.