"Here is what you need to do to create your own megaprims:
In Preferences/Phoenix/Build, check "Enable 64m prim support" and click Apply.
You will need to join the group called "Mesh Volunteers" - its simple to find using your search tool under groups, or go to secondlife:///app/group/3aa8389a-b658-7fb0-d0bb-75001a13db86/about/
Then, open this page - http://wiki.secondlife.com/wiki/Mesh/LiveVolunteers - to read up on Mesh support. At the bottom of the page there are several mesh sandboxes listed which you can teleport to where the megaprims can be created.
Once created, don't forget to name your megaprims using the conventional method "XxYxZ" so that you and everyone else can tell what size it is. Take them back into your inventory and now you can rez and use them anyhere in SL that allows megaprims! You may also edit existing megaprims in these sandboxes, but we imagine you will want to make your own.
That's all there is to it! Now we can make all of those oddball sizes we always seem to need"