Oh wait lol, I know the answer to this question, sorry, I have been slightly disconnected to BTB due to the $%^*#* license and I didn't click at your question.
The limit to an SObject is a mix of common sense and logic. You don't want the object to be so long that the game has to draw the whole thing over a 5 km track for example. There is no real exact figure to measure it at, the common idea is 'not to make it too long'. "How long is a piece of string?" is about how long it should be.
The best way to calculate something like this would be to understand the common lod's in the game. In most cases BTB sets lod's at 2000m, which is probably too much from my pov, 500-2000, depending on the situation, ie; if you have a sharp corner, no need to draw some parts until your almost on top of them, 2000 is 2km's which is quite a long way.
When it comes to SObjects you have a string of 'small' objects, but the way BTB makes the SObject, it becomes one gmt (object) in the game. SO if you have an SObject that is 5km's long, with a lod of 2000m , you are drawing that entire object when you are miles from where you need to be able to see it.
So, in this case with a lod of 2000, you want your SObject to be no longer than 2000m. But, if you were to drop the lod to 1000, you would want it no longer than 1000m.
If you have a very complex object with lots of polys, then you need to set the lod's as low as possible so as to see them when you are supposed to se them without them popping up like magic in the view range. Then the length of the Sobject would be determined by the lod you set it at. And in this case I would highly recommend making lower lod models for the actual object itself, which would then once again decrease the length of the SObject.
Does that make sense? It's not just 1 factor involved in the equation, there are a few that need to be considered
I think the best way to give you a precise answer is to actually see the object you want to use as an SObject. If it is complex, then it might be best allroud to not set it as an SObject and use it as a single individual object. ie; if it is a fence, make the sections in 100m lengths and join them together manually.