I haven't done summer pruning before either. I just saw a link from Dave Wilson Nursery in Central California that covered that. I'm not adept at putting on pictures or links sorry. Just google them and look at backyard pruning. The narrator goes through 3 years of growth which gets you most everything you need.

I will put a link to the videos. I never heard of this cherry, looking it up it sounds like an awesome tree! I want one now! Yes I would take about 1/3 of new growth out. Cut central leader down to make a open centered tree. I'm not sure you want to do that with these trees though? You can, but it may grow fine as a bush like cherry. You can prune it to whatever size you like. And yes for cherries late July when it is very dry is the best time to prune. You can prune in the spring but risk fungal infection., I spray with copper before and after pruning. I live in an area where it can be humid and fungal infections are a concern. This looks like a nice tree. I do really want to try this one.

The Romance series is a new series to USA from Canada and they have a similar growing habit. I'm going to ask about pruning them, you may want to follow that thread.
Still watch the DWN videos!
The bottom videos are the oldest, start there.

Here is a link that might be useful: DWN videos

Greetings:

I have a Mesabi cherry in my tart cherry collection. It was an accident as arrived at work on a load of container trees but was not ordered. Since the price was free, I took it home.

I have had it in the ground for several years now (came in a #5 container). It does grow much slower than my
other tart cherry varieties planted bare-root. Perhaps from
starting out as a container tree?

My Mesabi is now about 6' tall and really went into a bearing mode. I do wish it would grow bigger but maybe this is a blessing as easier for me to net the tree when the cherries are ripe.

I have not done much pruning on mine since I planted it as
it has been so non-vigorous in growth.

I also have North Star grown central leader (coming down as wiped out by bacterial blight), Sure Crop (leader removed for better height control) and Balaton.

The deer ate most of my young Balaton last summer so
back to being 3' tall and branched. It is now surrounded by
chicken wire to keep the deer away and hopefully get a chance to grow. My daughter's favorite tart cherry variety.

The Balaton is one MSU brought back from Hungary. They have 2 or 3 more now too.
Yeah I'm not sure you need to prune this one. It can't hurt to head the branches back as that will thicken the branches, and such. I have found that the technique to trim back to allow roots to develop really works well in making a strong thick trunk and scaffolds.I had to cut a tree back to a stub twice.First time was develop a new central leader, the original only had one branch. the 2nd was because of extreme winter kill. When i think about it I cut this tree back three times. And each time when it grew back the branches were that much thicker. it convinced me that pruning does help establish a tree.I'm hoping I don't have to anymore! It is a peach tree, and well going on the third year and no fruit yet. But it is a very strong healthy tree.
I learned a lot too!