Making a telescope for a pirate costume
A retracting telescope typically has 3 or 4 segments that retract into one another,



although the insecure pirate may feel the need to get a longer one.



The key features of the section diameters is that they slide easily past one another but will not fall apart when extended. In other words, you need a "stop" that prevents the larger segment from completely sliding off the next smaller one that's inside.

Assuming you have a method for building that stop, the tubes should ideally have a diameter difference of 1-10mm. A set of tubes with larger size gaps can still be used, but the extended telescope might look sloppier (the cumulative gap will cause more drooping, as in Jack Sparrow's overly long telescope above).