A quick way to estimate what will fit is to figure you want around 2" between the edge of the table and the track, so take the width of the benchwork/train table, subtract 4" (2" on each side of the table). That gives you the diameter of the circle of track, divide in two for the radius.
So 5 ft. wide = 60" wide; 60-4=56". Divide by 2 = 28" radius curve.
Now you probably don't want the track too close to the edge, and you might want to allow another inch or so for scenery so you might want to go an inch or two tighter, but 28" radius would be the absolute largest you could fit in. 26"-28" radius should work for pretty much anything you'd want to run. Walthers passenger cars require 24" radius curves, even big steam (BLI 2-10-4) will go around 24" radius curves.