I think a lot depends on which climber and how you train it. With most climbers you want to spread them sideways as much as you can to encourage laterals and therefore more blooms. In that case you need to allow for that sideways spreading. If it is a very large, vigorous climber you will need more room than for a smaller medium sized climber. Take another look at those climbers at the garden and see if the roses on either side were getting shaded out by the taller, larger climber. That may be why they didn't thrive. Also, by size alone climbers need to have very large root balls to support the top growth. They may have used all the nutrients and were water hogs forcing the smaller roses to die off.
