When I use the verb 'быть' in the present, the predicate takes the nominative case, as in 'Я - высокий.' But when I change the tense, I sometimes feel I should use the instrumental instead (so 'Я был высокий' vs 'Я был высоким'). In what cases do I use each?
In short, your question seems to be a duplicate of the other one that had been previously asked.
Quotation from my answer to the previous one:
The difference is that быть + Instrumentalis is similar to a past perfective form (as a perfective action unrelated to a moment of utterance), while быть + Nominative would be compared to a present-perfective structure in a language where such a distinction exists.
As for the future tense, some decades or a century ago the phrase with Nominative compliment [будет журналист] might be equally correct, but nowadays it sounds as a little bit obsolete.
The usage of Instrumental as a copular compliment is similar to that of Essive (a case of an impermanent state) in Balto-Fennic languages and can be regarded as a Fenno-Ugric substrate.
Я был высоким is grammar correct option because we can ask
я был (в прошлом) кем? - высоким человеком and instrumental case asking the same questions
But the first option also can be used in speaking and writing, so it's not mistake just nuance. Also you can use short form like:
Бунин был смел, честен в своих убеждениях.
As you can see there is no ending at all
смел (ый / ым)