In the AC2K help files, look up key word "Specifications" to see the limits on tables, recordsets, databases, etc. There are several pages that will help you with regard to sizing limits.
With good archiving and call details retirement on a regular basis, and perhaps with links to those archived tables, you could do some pretty good long-term record keeping. By linking to external tables, you can overcome the database total size limitation, but there are still many other limits that would be quite formidable to side-step. I understand your reticence.
We decided long ago that our own call tracking system was going to blow Access capacity out of the water. What helped us decide was that our system had a nearly unlimited number of "touches" for which arbitrarily sized descriptive & narrative fields had to be stored. Problems for us could be forwarded from rep to rep to take advantage of each rep's specialty skills. The detailed call history had to be tracked to help determine cost breakdowns by problem type, caller region, and rep ID. So we finally chose a commercially available package based on an ORACLE server but with a custom front-end. Straight VB, I think, but it's hard to say.
Of course, since our site is part of the U.S. Naval Reserve HQ, our user base and call volume are pretty high. We have at least 20 reps locally on-line 7 x 24 plus maybe another 20 reps during prime shift plus a dozen remote sites that take initial calls and forward them to us for subsequent resolution. So in our case, Access was never going to be adequate.
On the other hand, in a smaller shop, call-tracking isn't so massive a problem. The factors that will define how much storage you will need relate to the number of reps, number of clients or customers, number of times a single call will be referenced with a follow-up reference (if any), the number of possible call topics, level of detail to be tracked in your narratives, and how far back you need to keep things for historical purposes. Good luck in your sizing efforts.