Hi Paul,
Woody is right, some manufacturers use a standard 52mm tube for the short and then either a 55mm or 60 mm tube for the long one.
The only thing you have to watch out for is if you use the long tube for the nottom is you will have to press the mechanism in further than you would normally. (The refills only come in a standard length for some reason
)
Its always a god thing to measure your tubes from both ends, just in case one and of the tube is longer than the other.. 