Bob -- You don't have to worry about frost in Sunset 14, CA.
And yes. 5-Gal at minimum. 7 Gal is better.
In Southern California, many Miniature Roses can grow 6 ft tall, if planted in the ground.

Jeri

The bigger the pot, the better off you will be. Remember, the only thing miniature about a miniature rose is the bloom, the bush will be just as large, over time, as a full size rose.

I don't usually put a tiny mini in a huge pot. I use an intermediate size till it is a bit established. It works better for me.
Kathy

Bob, I'm growing out a couple of Poulsen Parade roses in pots. A couple of things come to mind.

First, there are about 4 or 5 plants in that little plastic pot you bought. You don't have to but you can separate them if you wish. After the way mine grew out, I wish I had separated them. They quickly grew to 18 inches.

Second, I haven't found my Poulsens to be particularly great in the sun. They open fast and fade to colors that aren't beautiful. They are bred as greenhouse potted roses to be sold in the bud. So don't be disappointed if they aren't the most beautiful color once exposed to California sun. Mine are light, subtle colors, so you might have better luck with strong yellow. They also had a lot of blackspot at first, but it has subsided.

Third, if you separate the plants, I'd pot them up first to what's known in the nursery trade to a 3 gallon pot and later to a 5 gallon pot. The best thing about pots is that you can move them around to the ideal location for the rose. So if you find these roses need a bit of mid day shade, you might have a location with filtered light under a tree during the hottest part of the day.

Frost is no problem. Freezing can be (but probably won't be) different. If we have another very cold December freeze like last December, with sustained lows down in the mid-teens for more than a week, you will want to make sure your pots are well-watered. If you're worried about them and it makes you feel better, put them against a south facing wall of your house until the cold snap is over. They will be fine.

Thanks to all of you who responded for your very helpful remarks. I knew I could get the straight stuff here. Berndoodle, your suggestions were especially useful, and I will put them to good use.

Bob