First of all, “link description” is defined in the database structure as VARCHAR(255), and I don’t think there’s anything in core limiting the size of the field farther. Are you sure that you can’t fit more than 30 characters in your bookmark descriptions?
That said, 255 characters can still be small if you are trying to fit several different language translations into the field. However, there’s nothing stopping from you from simply redefining the column type in MySQL so that it can hold more data. Defining that field as “TEXT” would let you store up to 64K in it, and you wouldn’t have to modify any core files to take advantage of that change.