While I don't disagree with the remarks of Donald - I don't think it's feasible to create really uniques descriptions for each type of product - especially when it's only the dimension which is changing (like the length you mention).
I think you mainly have to options:
-
keep the duplicate content (which is not necessarily a cause for punishment: check https://support.google.com/webmasters/answer/66359?hl=en), especially if these pages currently are generating traffic & from technical perspective it's not feasible to use one page where you can select the changing dimensions
-
use canonicals to a type of category page - which contains the general description with links to the products with different dimension
2nd would be my preferred solution - but again if not feasible I would just live with the semi duplicated content (we have some sites in this situation which are just doing fine). It's maybe a risk - but I guess it's not really a big one. You're not the only one in this situation - e-commerce sites selling common products like batteries or replacement parts are in the same situation as you. Many of them do just fine keeping the "duplicate" content.
Dirk