Small diameter vessels are characteristic to many tropical tree species but, I would like to know whether, in general, small diameter xylem vessels are shorter than large diameter ones in tropical tree species?
the active xylem section may be made by some large vessel or by a buldle of fine ones, and length may be a result of adaptation but you may check a specific answer within the refs of a metaanalysis by Jacobsen et al. http://www.ncbi.nlm.nih.gov/pubmed/22965850
In some families the observed fine xylems are xylem elements (cells not tubes) arranged with an overlap to allow the xylem sap flow so their length is very short at all.
Usually in more evolved plant families there is a tendency for xylems to become larger in diameter and length. The same is true during the development of plant in which first fine xylem tubes (proto-xylems) are seen. Later developed xylems after cease of the longitudinal growth are large in diameter (meta-xylems). However despite the difference in diameter both are considered xylem tubes that usually extend from leaf to root equally. Noteworthy that in a annual plants both types (proto and meta) are seen next to each other in a vascular bundle, while in a perennial plant the proto-xylems are seen only in current year growth and new shoots and they couldn't be found in other parts and trunk.
Therefore, back to the question, in my view we cant say that the more the diameter of a xylem the longer it can be.
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