<p>I'm trying to find out how much memory my objects take to see how many of them are ending up on the <a href="https://learn.microsoft.com/en-us/dotnet/standard/garbage-collection/large-object-heap" rel="nofollow noreferrer">Large Object Heap</a> (which is anything over 85,000 bytes).</p>
<p>Is it as simple as adding 4 for an int, 8 for a long, 4 (or 8 if you're on 64 bit) for any reference types etc for each object, or are there overheads for methods, properties, etc.?</p>

Don't forget that the size of an actual object doesn't include the size of any objects it references.
The only things which are likely to end up on the large object heap are arrays and strings - other objects tends to be relatively small in themselves. Even an object with (say) 10 reference type variables (4 bytes each on x86) and 10 GUIDs (16 bytes each) is only going to take up about 208 bytes (there's a bit of overhead for the type reference and sync block).
Likewise when thinking about the size of an array, don't forget that if the element type is a reference type, then it's only the size of the
references
that count for the array itself. In other words, even if you've got an array with 20,000 elements, the size of the array object itself will only be just over 80K (on x86) even if it references a lot more data.