As you have found on the summary page you can move the divide between the side bar, list on the left and the rest of the summary page. If you set up two columns under preferences – summary page there is no way to adjust the width of these columns one to the other.
I believe there is a minor resizing bug that I just recently noticed. On my laptop it needed several tweaks, and I finally isolated it to something that I rarely keep on my Summary screen, but was added when I got the new laptop and I did a fresh install. It was the quotes section, which seemed "wide" and was preventing proper resizing.
I also noticed that Reminders that are long winded (mine was "Davidson County Property Assessor - Property Tax") was not truncated. I changed it to "Annual Property Tax" and now it behaves better. Unnecessary wordiness is not appreciated!
Been having this issue for a while, where the rightmost Reminder column was off the screen and had to scroll across to find the $ amounts for the Reminders. I think I may have just solved it, for me at least, under Preferences, Summary Page, I removed the Budget Bar and Expenses graph (can't recall which of the two was a very long colored bar showing expenses by category for month). By removing that long wide bar, it then resized the rightmost column to much smaller and everything shows nicely on my screen now,
Note: if you are not already running 2021 it will only be a free upgrade to users who purchased a 2019/2020 key not to users who purchased a 2017 or earlier key. An upgrade for these uses is at a 50% discount.
Click FILE (MONEYDANCE on Mac) – PREFERENCES – APPEARANCE tab and set FONT and NUMERIC FONT to ARIAL NARROW using the Dropdowns.
Font could help as derekkent23 suggests. I also think there's a lot of sub-indents on column 1 in the Bank section (can't exactly tell) but this is where my error was coming from. The more words you have in things like security names and account names, the wider it gets.
Also in your sidebar, you have it set pretty wide - if you narrow that it will make more room for the 2 columns.
The key for me was to shorten naming conventions. I used to have things like
Wells Fargo Self-employment IRA, and
TD Ameritrade Self-employment IRA
Now they are WF SEP and TD SEP so my sidebar is narrower. Also, changing long names in securities (mutual funds are notorious for 5 letter tickers and long names) can help too. I went back to some abbreviations for them.
The more digits you have in the currency column also takes precedent. If you dynamically adjust the Sidebar, you'll see the "words" are changed to ellipses, but the numbers stay. So you can see where you can trim words to get a cleaner presentation.