When turning a bowl, what’s you go to tool rest length?
I haven't seen where you added more detail to your question, the point of the silly picture of the tiny bowl.
I still think there are too many variables for one "go-to" answer, at least for me. What's your go-to tool, your go-to grind?
My go-to rest is what ever works best for that turning. Outside or inside? 4" or 20" bowl? End grain or face turning? Shallow or deep? Shape of the rim (flared out, closed)? Turning with finesse or prone to catches? Production speed record or relaxing, zen experience? Species - basswood, cherry or osage orange? Soft, wet wood, twice or once-turned? Hard dry wood?
I have a variety of tool rests. More important might be the tool: the diameter and exposed length of the tool shaft, the grind, the length of the handle, body support and motion. Rough or finish cut? The depth of the cut makes a big difference as well as the turner's experience and tool control.
And the type of the tool - bowl gouge, carbide, scraper??? I sometimes shape almost the entire bowl, inside and out, with one or another Hunter tool with fairly short handles. I often switch tools (and rests) to experiment during the turning. I'm retired, don't turn to sell, don't get in hurry, primarily turn dry wood.
So my "go to" tool rests are 4", 6", 9", 12", 14", some Robust comfort or low profile, some BWT modular, box rests, and some specialty custom rests. Depending. All are within arm's reach and take only seconds to swap. Maybe I have too many tool rests - if I had just one I'd use it.