if you approach a reputable builder, they may be able to shed some light on tubeset spec based on telling them more about yourself, how you ride, and your physical measurements.
that might be a question for you and your framebuilder to hash out if you haven't developed a preference. design, construction (tig'd/lug'd/fillet brazed), your size/riding style, etc. all factor into the equation.
that said, I've been very pleased with the columbus spirit-for-lugs a/k/a "pegorichie." i have a few bikes sporting those tubes and they give me a good feeling, especially for cx. it's also available in "uber over-sized" now . .. some great options out there now for custom steel. have fun with the process!
Brent Steelman of Steelman Cycles is incredibly knowledgeable about various steel tubes sets and is also a terrific frame-builder. I've had several of his bikes over the years, most made of Dedacciai or 853 and they are the bikes I return to after being disappointed in other frame materials. I know he's using some other stuff as well these days. You should check out his website.
My new 'old-school' MTB was built out of OX-Plat (from TrueTemper), great steel.... one worth considering investigating.