Theorically 1 character = 1 byte, though that for a database it is not especially so simple.
So one thumbnail = 1 byte x the number of characters used for its code.
Thumbnails with "<a href...>" take more space than those with "[img...]" but I do not think that this makes a big difference anyway (though that I could do a test: convert all the <a href...>" and see how much we spare).

I do not think that old comments should be "hidden" like archived ones: often these old comments contain thumbnail or useful info that visitors "should" see. That's why I suggested option 4, as these comments are useless for the visitor in most of the case.

The advantage of option 4 vs option 3 is that the archived comments do not have to be displayed on the same page as the other, so the fact that they are stored separately is not a problem.

But anyway it is not technically difficult to have older comments stored separately but displayed with the newer ones, it will just maybe make some pages a little slower to display (though that the difference will still be measured in milliseconds, or in 1-2 second in the worse case I hope).

The big advantage of the option 3 is that "archiving old comments" is a computer-task, option 4 is a human task