Hi Tom,
Correct me if I'm wrong, but it seems you may be answering your own question: "My temptation is to make prints at the size I want to make them (within reason), and continue from that point...".
Much of what one sees exhibited in the commercial galleries, today, are very large prints, where photography is made to compete with other media, such as painting. Some photographic genres might be fitting to such overblown and monumental proportions, most, I feel, are not. My belief (very subjective on my part, here) is that there's something incredibly special to looking at a fine photographic image up-close. It makes for an intimate, jewel-like, personal and lasting experience. I agree with the comments, above, where portfolio series are made to be handled ("in-hand"), and looked at, print-to-print. I've seen so many "big-print" portfolios, where the size overtakes the content of the photographs. They're difficult to manage, not to mention, expensive to produce. I also think it important for the photographer to decide (manage) how he/she desires the final image to be consumed and experienced -- this will often dictate the size. Printing to a standard size (by that, I mean your standard, and not available standard paper sizes) might be a good way to start, giving you predictable darkroom workflow results. Your audience(s) will get to know you for that. You can always deviate from that, depending on the end use (exhibition, home display, limited edition portfolio, etc). I hope this helps.
All the best,
Andrew