...train of thought...
-Windows OS manages program size and positions.
Normally, you shouldn't have to resize and use 'crtl + close (X) to retain the size and position of a program window.
Windows OS will retain the last size and position by merely closing a program on the desktop.
This is normal windows behavior as you know, and see it everyday with your other programs.
(unless the properties of the shortcut/app is set to run minimized or maximized.)
It is the same for JRiver, there is no difference.
But for you it is not. Why..?
As a guess.
-sometime in the past an utility was used capable of resizing program widows and positions.
-an entry was made into the registry for the JRiver program to open in the cuurent state.
It is possible, this utility was uninstalled but the registry entry remains.
if a clean/fresh install of the OS after experiencing this problem was never made, it is probably still exists in the registry and still in effect.
i am certain most people will agree that you will not have this problem with a clean fresh install of Windows 11 OS and JRiver program.
But i am also certain you would prefer not to do this.
what you can try.
-use the same utility (if you remember ever using one,) or get a similar one capable of making these window adjustments.
Search for free or trial period utilities on the net with keywords: winsizer autosize progsoft. progsoft net is in Japanese.
NOTE:
i have never used any of these utilities you may find nor am i recommending the use of them.
-please understand i am only guessing as to why Windows 11 is opening JRiver in a very small window.
-maybe a registry entry was made which overrides normal OS windows management
-maybe using the same or similar utility will rewrite this problematic entry.
But it may lead to other problems, i don't know. it may not.
if any of what i wrote rings true to you, it, at least, is one thing more you can try.
otherwise i am out of ideas.
Take the usual precautions before using these utilities.
PS
the irony is suggesting to use an utility that may have been the cause of the problem in order to fix the problem.
the best suggestion is to get help from someone else who is certain of what is going on. i am only guessing.