Well Bruce ... it really depends on what you are referring to as a "standard" radius curve. Having worked exclusively in flex track through the two nice group layouts I've been associated with in HO/HOn3 modeling, I'm not sure if there is a fixed "standard" radius for fixed track curves as produced by the major suppliers, but would suppose that if there is, it would likely be around an 18" radius curve.
I know most of my larger locomotives, being brass imports, had trouble handling curves at anything much under 30" radius, and so most of the major curves on our large layouts were designed at a 36" minimum, but I must tell you ... that takes up a lot of space, and even then the larger steam locomotives like my cab-forwards, or other articulateds, just don't look quite right going around anything tighter.
You must also give thought to your turnouts, whether for sidings along the mainline, or more especially in the yards where multiple turnouts come one right after another. The smaller the turnout number, the tighter the radius created, and thus the more difficult it is for larger engines with longer wheelbases to negotiate. Most large engines cannot tolerate the common #4 turnouts, and are even likely to trip up on a #6, especially when laid in series. #8 & #10 turnouts are usually best for longer wheelbase engines and articulateds, but they also eat up a whole lot of room.
BTW: just in case you are unfamiliar with the concept, the switch or turnout number references the ratio of divergence of trackage created by the turnout, such that a #4 turnout is physically much shorter and causes the track to diverge much quicker at a 1:4 ratio, while a #8 turnout is much longer, and causes the tracks to diverge more gently at a 1:8 ratio.
The biggest question is how much space are you willing to devote to this endeavor, and then once that is determined you have your control limits pretty well set concerning the radius of curves and what size turnouts will allow you to fit what length sidings and yards in as well. Then you will be better able to determine what size and kind of motive power would best be suited to the space available. If you choose to design it the other way around, buy deciding what engines you wish to run, how well you want them to perform and appear while running, then you may be in for a rude shock when you find out how much space you are going to need, or worse yet, when you find out that they just don't "fit", nor work reliably, and end up having to downsize your motive power dreams.
Just as an alternative for you to consider that might help keep it all smaller and easier to do. Perhaps give consideration to a small logging or mining railroad, instead of a big class A mainline operation. Smaller and prototypically very slow geared locomotives are specifically designed to handle tight radius curves and tight turnout trackage. Their proper slow speed makes the layout "seem" much larger, whereas to run a mainline operation at scale speed means you need a whole lot of track just to get up to scale speed, and you'll find you are rolling through your whole layout in no time at all. Small industrial switching layouts offer the same advantages as the logging and mining type layouts, and as a bonus, should you ever get connected with a truly large group layout, your smaller setup may well be able to be integrated into the larger layout as well.
Any other info that I can provide, please don't hesitate to let me know, as I do have rather deep experience along this line!