As chibacity said, IDisposable is not related with LOH management. Here is a good article on LOH: Large Object Heap Uncovered.

That being said, there is not any public API to determine the LOH size, as far as I know. You can find a reference to the 85000 bytes limit in SSCLI "rotor" (source available here: Shared Source Common Language Infrastructure 2.0 Release Download):

in clr/src/vm/gc.h:

#define LARGE_OBJECT_SIZE   85000

Although this source is a CLR 2.0 equivalent source, not CLR 4, I don't think they have changed this, as it would certainly have deep implication on existing code.

So if you want to do something smart with this value, you probably can put it safely in a constant, or make it configurable, but it will certainly not change dynamically during the course of the running process.