NOt sure if anyone has suggested this yet, but ...
Why not add an item to ObjectProperties that will tell you how many prims it is BEORE you rez it. (like when you right-click on an object in your inventory and view the properties..on that, it could say '1 object - 250 prims'.) that way you will know BEFORE you try to rez it how many prims it is and if you can or prepare some space if you need to.