and how much of a difficulty change equals how much of a block size limit change. The baseline could be how much difficulty is at the time of the change (for example), but the other part is tricky. Maybe it could be done as a percentage change, it would change the same percentage as difficulty?
Or it could be a hybrid of your two ideas. The increase in difficulty triggers an increase in the blocksize, but not linerally. For example, no matter how much the difficulty increases (beyond a minimum), the blocksize increases by 10%. No matter how much the difficulty decreases (beyond a minimum) the blocksize decreases by 5%. Or vise versa, depending upon which is more likely to result in a favorable sscarcity.
Throw in my unlimited-if-all-free transactions rule.