It really doesn't matter so much the size, it's the shape that's the problem. No, the goal is not to keep it against the walls, it's already making maximum contact there, right? Can't improve on that unless we 'provide more wall surface'. That being the hot glass inside the load. Heat transfers too slowly though the glass to make being thick an advantage (another sphere drawback unless they're tiny). You want direct contact (meaning fine grind and light packing), at least that's what I found. And, I think, others that have tried it.
I usually use 7 flowers. Two on their sides on the bottom with a layer of bud over them (about 1/3 the load I'm using, about .030 grams for instance). Then 3 flowers, again on their sides, again pretty random. 'Where they fall' as long as they don't stick up too much. Then the other 2/3 of the charge over them which I press down a bit to get it below the line. The final to flowers go in last, 'thumbtack style' side by side. This also keeps the top of the chamber (bottom of the lower stem) where the holes are cleaner. You need to be careful closing it up, you don't want to jam a flower into the seal, but it easily becomes routine.
I dump the load into a mint tin while hot (it comes out with the flowers and all, no problems). I shake the box a bit to loosen the flowers from the ABV then shift the flowers to the lid for loading the next load.
The flowers are a few bucks, give it a try. Some complain of the hassle, but none I think haven't reported better clouds on less bud this way.
Oh, yes, the other insight from the testing is temperature recovery is slow, precise (within a degree or two, perhaps the best I've seen) but giving it a while between hits to recover full heat throughout the load (remember it only drops after you start to hit) is a big help. Try up to say a minute?
http://fuckcombustion.com/threads/ascent-vaporizer-by-davinci.9885/page-111#post-459922
http://fuckcombustion.com/threads/ascent-vaporizer-by-davinci.9885/page-262#post-656321
http://fuckcombustion.com/threads/ascent-vaporizer-by-davinci.9885/page-118#post-470034
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