for machs just make the radius large like 99 or for really big 999 in mach tag ,thanx Neodos.
For blocs change bounding offsets X Y Z in bloc tag to the same size you stretched its model,coll,and phomo to. You can also set the fade and dissapear distance in the Hlmt tag.
I know it's old, but a word of caution about this one: if you have too many objects with a high bounding radius, things will start to disappear, and your biped may lose collision. If that happens and you don't think you've done anything else to break your map, try turning the bounding radius down on all the objects for which you raised it.