from wiki:
smooth rods
measurements will depend on desired build volume. 350 mm X 320mm X 350mm
gives about 200 x 200 x 170mm
as for threaded rod it depends on build volume. since the threaded rods go along the same direction as the smooth rods on z axis i would say start off with 350mm - 50mm less for area obstructed by motor=300mm for z axis.
for x and y, there are limits to bed size, so 350 and 320 will work within the size of the bed, and probably will be too long.
imho, i hope this helps.
someone probably should specifically state the threaded rod size in wiki, it will make newcomers a lot less nervous.