Hi,
I am late to the party, but I was just searching for a related issue, similar to this one regarding small objects popping in to existence, but slightly different. The objects I am having an issue with are fairly large.
Anyway, I thought I would just post something I learned a while back about modelling small objects such as cones or markers. The reason FSX won't draw an object until you are a certain distance from it is due to it needing to be a minimum of 1 pixel with default settings. One way to trick FSX in to drawing smaller objects is to make an object such as a normal sized cone, but then include a larger box beneath the cone, and attach it with a small column or similar. When you place the object, you just adjust the elevation so the larger block is buried and can not be seen, and the cone appears to be sitting on the ground as normal. This way, FSX is fooled in to drawing the cone from further out, because it sees the model as larger than 1 pixel sooner.