I recommend that you post your question in the Shrubs forum her on Gardenweb - someone there will be able to give you advice on this.

Did you feed them? Something low in nitrogen and moderate in potash so they harden up for winter.

I'm guessing that, as they've come from the supplier, they've been pruned back for planting so the new shoots don't wilt.

So in spring, after the risk of frost is gone, I'd prune with secateurs and take the new growth back to about two leaves from where the new wood starts. Photinia is very good at back budding and putting out new growth.

The secateurs ensure you don't end up with chopped leaves.

Later on, when it reaches a greater size, it can be cut with a power trimmer, although it will probably look awful for a few weeks until the new shoots break out.