Settings > Preferences > Miscellaneous > Advanced > Limits:
The limits of these values are 4294967295=2^32-1 or 0xffffff (seen in the "userprefs.oxc"). But it can't be true, e. g. the limit of "max_thumbnail_size" is 4096 px how I found out. What are the limits of the other values? There's no information in the manual.
Thanks in advance.
What actual problem are you trying to solve where you need to know the exact limits of those numbers?
Oh my gosh. This is a mean question, don't you agree? 
Well, I would like to calculate the MD5 of ISO Files of Blu-rays and I'm not sure if the limit is enough.
And because I'm a fan of Directory Opus and I'm curious to know the limits. I know curiosity killed the cat 
so I hope I don't create you extra work by putting this question.
The question was aimed at trying to understand what you're actually doing, because the real solution may be different to just putting a larger number into that field.
(Or, where it is just idle curiosity, we have more important things to do than investigate trivia for you. No one is going to want thumbnails 4294967295 pixels wide; you would run out of memory. Similarly, no one is going to want the zip double-click time to be greater than 8171 years. Let's be real here.)
The MD5 size limit only applies to automatic calculation of MD5s when the column is turned on. It is there so that if you have the MD5 column visible and go into a folder containing huge files, you don't end up wasting a lot of time and HDD/SSD/network activity calculating MD5s for huge files you may not have wanted.
If you use Edit > Calculate MD5 Checksums (which runs the GetSizes MD5 command), the limit is ignored and it will always calculate checksums for all selected files, regardless of size.
If you really want automatic calculation of MD5 checksums, no matter how large the files are, then you can set the limit to zero to disable it entirely.
