Photinia Pink Marble - Pot-grown trees

Hello, I'm new here and new to gardening generally. I just bought two pink marble photinias that have been grown in pots in a tree shape. I was planning to put them in two pots I have by my front door. They have arrived and are a little bigger than expected - in their pots, floor to tip they are about 5 feet tall. The pots themselves are maybe around 25cm across the top.

The pots I'd planned to put them in are about 28cm or so and I now wonder if they'd be too small. I don't particularly want the trees to grow any bigger anyway, so is that ok or too small? I don't intend to ever plant them out but rather keep them in pots for as long as I can keep them alive!

If anyone could give me an answer in the next half an hour or so it would be much appreciated as I was going to pot them this afternoon and if I need to go buy more pots I'll have to go soon!

I also would like any advice anyone could offer on keeping them alive as long as possible! thanks!