I will cover everyone here on this and save multiple entries.
First @ nod114
Not advisable to modify the sizes of objects unless you are going to get the source and modify the physics. The engines I am discussing have prebuilt physics. If you make everything small an comparison to the terrain, you would find the speed you travel across the landscape to be extreme fast. In Multiverse it is not really that bad because the engine has very basic physics and no falling speed. However with Esenthel the physics covers falling, jumping and so on.
@lmaceleighton, wysardry
Multiverse has been around since 2005 it uses L3DT mosaics directly. Just plug and play an exported PNG mosaic with the alpha maps directly from L3DT. Multiverse uses the axiom engine which is a c# port of Ogre and uses the PLMS plugin to page unlimited terrain. MV is free but you have to use the portal to access the network. If you want out of MV network it will cost you $200K.
Esenthel has been around since 2007. If you are familiar with how to put maps into UDK Esenthel uses a similar method except the image size can be much larger than the 128x128 pixel limitation of UDK. The editor accepts any size PNG image and you set blocks of terrain which are then broken down into either 32x32 or 64x64 chunks ( I recommend making mosaics of 2048 size from L3DT export PNG. The chunks are independent of each other so the limitation I would guess would be +-2 Terra blocks or +-2 Terra x (32x32) pixels blocks or (64x64) pixel blocks. Beyond +-2 Terra I would venture to say there may be problems with mathematics of drive access. (as noted the limitation of terrain in Esenthel is hard drive related) Esenthel is free to use for non-commercial projects but 200 bucks for commercial use. If you want the source it is 10 grand.
why Esenthel over the others:
Esenthel allows you to edit the terrain as you design the world you can build rivers, waterfalls and so on without having to even close the world editor. You can put unlimited colors on the terrain if you have a fast enough computer (beyond the 8 texture limits of MV). You can punch a hole into a hillside and insert a model of a cave system (use L3DT to make several terrains and flip place one on top of the other). You can place houses into the terrain and have basements that lead to underground dungeons. (the engine can make true streaming unlimited seamless worlds!)
Esenthal converts just about all the common model formats directly. You can build collisions directly from static meshes. Since Esenthel supports common model formats Blender can be used to develop all the models which means no investment for $3000 programs. The set of tools that come with this engine make it a AAA capable engine for the hobbyist.
@wysardry
I don't know why you need the source code for this engine. You have complete control over it from C++. The only thing you can not do is custom shaders and objects which is beyond most game programming.
For a first time game I would suggest just using the basics. If you need Networking I would recommend Netdog over RakNet. I realize RakNet is free to use but it is designed more for Multiplayer the MMO whereas Netdog is a MMO networking engine. For 300 bucks you get 1000 CCU (concurrent connections) which is about 10K player base. If you grow beyond that just buy more licenses. Unless you are a large company with a Hyped game you can start at this level and alway work your way up. If you ever need a unlimited licese it is 150K but you will have the money to throw at it.
Demi