Rose advice please

Hi,

I'm looking to have a pair of climbers (new dawn) in separate pots on either side of my window at the front of my house which is north west facing. The pots will be set into a wooden planter I intend to build with a trellis fitted on the inside and I intend to grow them to about 8 feet tall. I gather they need quite a deep pot for their roots, I've found one that is around 17" tall but only 12" wide. My question is, will the top opening of 12" by 12" be wide enough to accommodate the plant?

Thanks

Ok thanks.