What do you mean by mature size? I have several Trader Joe's mini roses, my biggest is about 2 feet
Carla in Sac

I got a beautiful red mini for $1.99 last year after Valentine's day. It had about four to six rooted pieces in the pot so I separated them and potted them up individually. They grew well in bigger pots. Did the same this year with a beautiful peach one. If there are multiples stems in the pot they do well divided up.

The grocery store minis are basically just newly rooted cuttings so probably about 3 years to fully mature like most bands. The eventual plant size will depend on the variety. Some do stay smaller and compact but many will get as large as a floribunda or hybrid tea. Jeanne Lajoie is a mini climber that can get 12 ft. tall! Mini only refers to the size of the leaves and the blooms, not the size of the plant. They won't reach their full potential in gallon pots though. If you can plant them in the ground they will grow much better for you.

Trader Joe's has some unusual colors. Very close to Honey Dijon and Koko Loko. You can't go wrong for $5, and they're a very nice size.

Thanks for the answers so far folks...today more plants have buds, very nice.

I know these roses are pretty much just little cuttings when you get them in the store- by mature I mean mature enough to plant out in the yard and have them actually survive. An adult plant rather than a baby cutting.

I don't plan on keeping them in gallon pots forever, just want to grow them out to a mature size to give them better luck at survival. They will likely end up in bigger pots before too long. I'm used to growing out perennials for a year or two to a more mature size before planting out- this is just the first time I'm trying it with the mini-roses.

If it can take around 3 years to get to a nice maturity, now I can plan on what size and space in my perennial nursery will be needed for a year or two grow-out.

And Trader Joe's is a no-go for me. Outside my travel zone, and I can pick up these roses from other retailers more local.

So it sounds like instead of the size of the growth, you should look at the roots to determine when to put yhem in the ground. I definitely think one gallon is fine for the first year, these little starts don't grow that fast
Below is a photo of three I put in a pot I think 4 years ago(pardon the weeds, still working on refreshing the plants in pots!)
Granted they were living more in "Motel 6" instead of the "Four Seasons" and not fertilized much, but they don't grow that fast. And it should be noted they ALL blackspot horribly until the dry season. And with the exception of my red and white striped one, they all changed colors once out of the hothouse.

Carla in Sac

Here is my four foot mini when it was just starting to bloom next to Sarah Bernhardt peony. I've grown it about 10 years. Diane

Oh my, those are all so pretty! And all do look like they get to much bigger size than their baby cutting size. Not a bad thing at all..... I was hoping that they might grow up kind of bigger like that.

I'm honestly not sure of what colors I have or not anymore- between die offs and getting some tags mixed up I've lost track. But the fattest buds on a couple plants are either the white or yellow I think by the looks of it- I think it might be the whites because those had really large blooms originally.... the rest of them are still too small and tight to tell yet.

So, with the name mini roses- I know that's a bit of a misnaming, since the plants aren't mini. Kind of like some folks assume cherry tomato means small plant because small fruit. What are these roses in more formal naming then? That grow to be large plants but with mini blooms?