You'll have to consult the manufacturer for that info...on their website. Generally, 22" is the minimum for four (4) coupled and flanged drivers in HO with NMRA RP-25 wheels. The BLI 2-10-4's need 24", according to the info I have.
I have a Lionel Challenger, and Lionel maintains it is good for 18", but "recommend" 22" and up. Frankly, it doesn't look great on my 22"ers, either, but that is what it has to run over. It does it very well.
If you can manage it, your bigger locos will look terrific on curves greater than 28".
One final tip. My Challenger found every dip and kink in my track, and so will your Big Boy. Make sure your track has no dips from join-to-join, and that transitions into and out of grades are silky smooth if you have any curves nearby. The problem comes when the front truck and engine get around the curve, but are being lifted by the kink or abrupt transition. While this is happening, the rear engine wants to stay flush with the boiler in its "detent" or neutral position, and becomes misaligned with the curve when it gets to the point where the track rises to meet it.