Welcome to megaprim search engine
Second Life™ does not allow creation of prims wich are bigger than 10m in any dimensions. This can be very limiting for building large structures. It's also using up more of the limited prim resources one has available.
Due to the bugs in SL server software it was for a short time possible to create prims that are bigger than this. These prims cannot, however be resized, so one can use only preset sizes that were created while the big prim creation was allowed.
Originally there were only a few sizes available, but with the introduction of Havok4 simulatiors, the bug was reintroduced, which allowed for a short period creation of new sizes. Many people have used the opportunity to create them during this period.
Zwagoth Klaar did a great job of collecting and organizing many available sets on and making them available for free. This is search engine's goal is to make it easy to find the sizes that you need.