Everyone at some time asks for advice on what is the "best" radius for curves in a given scale. The question is only answerable by the asker. Strange, huh? The reason is that the person asking is the only person who know what curves he needs!
Wait a minute,...that's no help at all!
Ah, but it is. The person asking will have his/her answer when he/she knows the answers to questions he/she ought to ask first. Questions such as:
What scale am I using?
What types of rolling stock will need to be able to negotiate my curves?
Can I fit curves of that nature in the space I have allocated and still complete my intended track plan?
What would I need to change in my ideal and available curves if I were ever to add new rolling stock and locomotives to my stable (yes, almost all of us do add new items eventually)? Would I need larger curves, or could I keep the same ones?
Our prior preferences in rolling stock, and then based on the era and prototype's use of rolling stock, and then again what the manufacturers offer as facsimiles of those items, determine the nature of our track plan, including the curves we'll need. There will be a lower limit in radius until things start to go wrong. Upward to expanding radii should present no problem or danger provided you can still shoehorn them all into the space you have, or into the same track plan if you hope to keep it.
In the end, once you decide on what you will purchase, laying smaller and smaller curves with flex track, and then testing coupled cars and locomotives through those curves, is the only sure-fire way to settle the matter. Even stated specs from the manufacturers are sometimes iffy (ask me what I learned about the Walthers heavyweight passenger cars with their diaphragms).
Finally, if you want to err on the side of caution, add 10% to whatever lower limit in radius you find you need. Especially if you are laying curves out by sight and not using strict measurements and centerlines. Or, just use the broadest curves you can "afford" to lay and still have good use of the space you have to play with.