How to accurately specify the size of the Bootstrap column?

with a certain marginal and zeroing the last

\rhttps://getbootstrap.com/docs/4.1/layout/grid/#no-...

does not fit and moves down

give code, bootstrap has nothing to do with You did something wrong, not your call to move

How to set a specific size for the speakers?

or not to use a grid and using px, which in my opinion would be a very odd decision or to understand how the grid works:
\rhttps://getbootstrap.com/docs/4.1/layout/grid/

I am sure that You are correctly building the code, hence the frequent problems. Or don't use row or containers are not correctly used. Give code - prompt. If all accurately to do, problems such as You do not exist.

99% I bet that you have pictures of different heights and it can be even a fraction of a pixel.
Wysota block will not save you, because when you put picsom you will have problems with different lengths of content especially when adaptive.
1 option is pravelno to do previews and pixel to pixel.
2. to avoid 100% this situation in the future can make a small addition in php
Yes, it is not in the layout but all the same
and so you will need to do
Number 12 is empty and visible-lg in your case every 3 div
well sootvestvenno number 12-gel MD every second
what it will give
1. Divas will never crawl what would you not do
2. the height of the DIVS will adjust automatically
3. only standard classes and no Ls for this
4. stability adaptive
5. if the divas will be more than the standard then this change will only affect one row
an example you can look here
\rhttps://mydefile.ru/catalog