You would need to be pruning it an awful lot and the plant would NOT be happy being confined to that small a container for more than a month or two. This is a shrub with a tall, fountaining habit to 5' or more and as wide, if not wider. Here, 8'x8' is not uncommon if left unpruned.

Would a smaller, dwarf form of purple leafed barberry work for you? Something like Bagatelle or Concorde or Admiration? Or a purple leafed heuchera?

brushing up against.. or falling into a barberry in a drunken fit.. is not something i would find pleasant on a balcony ...

one of the meanest plants out there.. thorn wise ... though maybe there are some thornless types???

dad had a couple under my basketball hoop when i was a kid ... i hate these plants with the passion of a million burning suns ...

ken

The dwarf forms I referred to are neither large enough nor thorny enough to ever be a concern.

I have Berberis thunbergii "bagatelle" growing as bonsai in a very small pot, and it's a strong grower so it needs frequent pruning. I chose "Bagatelle" because of its tendency to form a trunk. It seems to do fine with a repotting biennially. I'd guess this plant is 3 or 4 years from purchase date. Foliage is deep/ dark maroon - almost black (thus the pot) and quite shiny.

From what I read, "Roseglow" is a slow grower, so you shouldn't have too much trouble keeping it pruned in whatever topiary shape you like. It might be a challenge when it comes to repotting, though. The short trunks on the 2 I have provide a good grip while repotting. Your plant won't provide that. "Bagatelle" came from Pam's (GardenGal'S) neck of the woods, Iseli Nursery.

To keep your plant in a small pot over the long term will require that you prune roots regularly, so that's something you'd need to be willing to learn to make it work out as you wish.

Al