I too would suggest 24-26 inches. Depending on what you tend to pack. If it is dense stuff (books, chocolates, nuts, stone/glass art, etc), lean towards the smaller size. If mostly light stuff, eg, clothes, then towards the bigger size.

There is also nothing wrong with having both 24" and 26", and picking one that is suitable for a particular trip. In fact this is what I do. I even keep a 28" leftover from the old 70LB days; it proved handy when I had to carry some unusually fluffy stuff (comforters, sweaters).

If you want to cover all contingencies with one bag, an expandable 24-25 may make sense. However, expandability has its own disadvantages. The extra mechanism means an extra point of vulnerability and failure. Moreover, if you use the feature aggressively and pack a lot, it can unbalance your bag too.