For me, the most important criteria, is what do I want the RR to do. for example, in my case, I wanted a lot of local industrial type switching. the rest of the layout would be designed to support that.
For others, the main criteria may be displaying their full length passenger cars in moving trains, or representing a portion of a double track or more main someplace from their memory.
Obviously this criteria is based somewhat on the equipment you have collected, but then you collect the equipment to represent what you want to model, don't you? so what comes first the chicken or the egg?
Era is important too, the modern the era you want to represent, the larger the equipment will be and hence the large will be the required curves. 4ft frt cars and 1st generation diesels or small steam probably do quite will on relatively small curves. On the other hand, 80 ft passenger cars and auto racks and big modern diesels need much larger curves.
type of RR is important too. A single train main with passing sidings takes up much less real estate than double or multiple track (ie loops). Generally in my experience, it takes about 4 inches of width for each track. So with a single track line, you can get more sidings and fun stuff (industries are fun stuff for me) in the space than you can with double track.
As those things are considered, then you evaluate what you can do within your available space. At this point you determine what you really must have, and what you can live without to accomplish some of the objectives you set.