I agree with Alvaro on almost everything mentioned in his answer.
Another thing to take into consideration, what if the actual equation was much larger/ complex, and there were way more elements that needed to be selected.
In that case, both of the approaches mentioned might become cumbersome and slightly messy.
Why not, give one touch/ edit button/ option. Clicking on this opens an editor with zoomed in text, and the user can start editing directly from there. Something similar to using android keyboard. When you enter an incorrectly non-dictionary word, it allows you to add it to the dictionary; but opens an editor before you save the new word.
In your case, the editor should ensure that the text size is larger than normal, for the user to easily select the element he desires.
[Please correct me if am wrong but my assumption here is that the only use case you are referring to is select an element and then edit the same]