You have probably heard different opinions about megaprims in Second Life, but don't be afried of using them.
The megaprims was created by some residents years ago (they found a bug in the client and created those giant prims before the "hole was fixed).
Linden has never supported megaprims, and there has been lot of talk and rumour through the years, but we still have megaprims (and I'm very happy for that). I know the The Havok™4-based Second Life simulator did some trouble.
Just never use megaprim larger than 256 m.
I have used them for 1½ year, with no problem. But I do make them all phantom. I noticed that people sometimes fly up in the sky, or was stuck (in the middle of nowhere), and that was because of the megaprim, but the problem ended from that day I made them all phantom.
A megaprim thinks it's only "10x10" m.
You can change them (almost) as normal prims. Just play a little with it.
You can find free megaprim many places in Second Life.
But you are welcome to visit PowerMatch (I have placed a big box with tons of megaprim within).
SLURL: http://slurl.com/secondlife/PowerMatc...