Hi,
you have reached MySQL hard limit, try to read here for more info and possible solution: https://www.easy-profile.com/support/mysql-issue-that-no-one-i-know-has-ever-uncounted.html#reply-4666

Also, you can try to remove trashed fields.

Yes, but keep in mind that fields like checkboxes store value in JSON format, so for example:
in easy-profile.com we have a checkbox field called "Interests" with these options:
- music|Music
- movie|Movie
- art|Art
- design|Design
the maximum value can be ["music","movie","art","design"] (max 32 chars).

So I can reduce the column from 255 to 32.

Thanks. I don't understand the way you are reducing the size with your example. Where do you define those maximum values?
All my checkboxes have these possible values:
0|NIL
1|KA50
2|MI8
3|UH1
4|SA342
5|BO105
6|MI24

How do I reduce this to ["NIL","KA50","MI8","UH-1","SA342","BO105","MI24"]? Do I have to change the type? Or do I have to use this second list instead of the first one in the options list?

In other words, how can I reduce the length of a given column to a customized value different than the default 255?

Hi,
sorry for late reply, it's too late and our offices are closed.

if you select all checkbox in your field the value will be ["0","1","2","3","4","5","6"] (29 chars), so you can change column length from 255 to 29 (50 is better because you can add some other option without problems).

How to edit column length?
- you need to use some tools like phpMyAdmin, normally all hosting provider provide some tool like this.
- search table called prefix_jsn_users
- for each field you will have a column with same name of field's alias
- edit the column, from VARCHAR(255) to VARCHAR(50)