Bigger the better, unless it cramps aisles or terribly limits operations. The plan for my bedroom-sized layout has a minimum standard of 30". That's barely adequate for most articulated locomotives, brass 2-10-2s, and full length passenger cars. I wish I could afford curves of at least 42" radius. But that's not practical and am forced to compromise. If you've got a whole basement, you can do better than I. Five times the maximum length of any rolling stock would be an ideal minimum radius. You'll probably want to compromise too.
Mark
Set up is going to be in a basement area so space really isnt an issue for me.
I am fairly new to this hobby, really a model car builder but I have ben wanting to get into trains for quite some time now. I recently purchased the train set I wanted. It's a Rapido VIA-Rail Turbo Train. It has 9 cars and requires a minimum 24" radius. My question is with al 9 cars will this radius be large enuff to look realistic or should I go with say a 28" or 30" radius when I build my track? I also wanna add a TGV Train at a later date so I kinda plan on having 2 tracks meet as one. What would be the best way to approach such a curve, Super Flex track? I plan on using Atlas HO code 83 track as well but they have no fixed curve pieces that are larger than 24". If 24" curve looks realistic with this train then the problem is solved I guess lol. Thanks for your time and help!