There's no such thing as "too large" a main stone in nano iwagumis, really. If it fits in the tank, it's good. A lot of the bulk is going to end up being hidden in either the substrate, or the plant growth. Keep in mind, this is only 10 days worth of growth. Over the course of the next few weeks and months, the carpet is easily going to eat that extra 15%.
If you've ever seen some of the old iwagumi nanos from the ADA gallery, (This video at 1:18
, and here
) most of the main stones are all oversized. Size can also be deceptive, take for example This stone
, pictured in my Mini-S. That stone is gigantic, it's almost 2x the size of the main stone I'm using in my current scape, it barely fits in the tank, but it still manages to look undersized. It's much better to err on the side of "too large" and letting the plant growth eat up any extra, than starting out too small and losing your entire scape.