What's the size limit of a prim?

I found I only can make a cube with the size limit as 10X10X10. But I heard the size limit is 30. Is it correct? How to create a prim with the maximum size ?

the limit size is 10x10x10 but you can find some mega prim by 10x30x0.5 or 20x20x0.5 etc but are last in inworl because is not allowed upload new mega prim, you can find it in many places in SL even for free just look on search or SLX.

slexchange.com has a free pack of them. i personally have not had any troubles with megaprims but some people have reported problems with lag ect so make sure you read a bit about them on these forums befor you decide to use them

hopefully one day LL will increase the size of normal prims, it will make life alot easier for builders

-Crunch

You can make a cube prim 10x15m too. Taper/Shear/Flatten.
Not that the OP asked...

The current game engine officially supports only a maximum of 10 M in any dimension. So normal building links multiple prims to make larger things. The limit of 30 is roughly the limit on center to center distances for a linkset of multiple prims, not the limit for one prim.

HDM prims, or Megaprims, are a special case. They were created by exploiting a no longer existing glitch in the building system. The existing megaprims can be copied, textured, tortured with cuts, dimples, etc... but can not be re-sized. Megeprims are known to cause lag and other problems in some cases - especially if they get too close to a sim border or overlap sim borders. It can be difficult to rez other prims on or near a megaprim. You can't link most megaprims to normal prims.

Used carefully, megaprims do have limited uses in saving on prim count. But they remain officially unsupported by LL, and if complaints are raised against them, the Lindens can and will remove them from a build without notice.