Well, the level size of 60.000 x 35.000 quants is ok. I don't see no reason to scale that down. If you've set up your physics code to work with that scale it wouldn't even make sense to change that now. Especially 281 quants in lenght and 113 in widht are not to much for an object.

WED doesn't care about your level size in terms of quants. But placing a plate that fills the whole level is a rather bad idea. You level should consist of several blocks and objects, not a single plate. The size of the compiled level depends on the number of blocks, the amount of lightmaps and their resolution and the number and size of textures used.

If splitting the levels up makes sense depends on the size of your level - not in quants, but in lightmaps and texture terms. If you got large amount of lightmaps used and there are textures that are used exclusively in special areas of the level, splitting these up can make sense. It terms of gameplay it is important, that the player does not get the feeling he is magically transported when loading level data. Small corridors with edges are optimum places to do a level load. Don't let objects all of a sudden disappear. Arrange your level so, that there is a limited field of view for the player where other level areas can hide.