If I were to go for a bookshelf sized speaker these days, assuming a smaller room. I would go for the JBL Studio 530 when they go on sale. Waveguides tend to be pretty forgiving of space and JBL just somehow manages to afford to get good performance out of even their budget line.
If the room would allow it, I would opt for 2.1 instead of trad 2 channel. I do both but my 2 speaker only systems tend to involve larger speakers, or 3-ways that will go down to at least 35hz for the music I listen to. On my smaller speakers, subs really fill in the bottom nicely. Right now I am listening to a DIY speaker designed in essence of the LS3/5a, but along with two 12" subs, and it is a marvelous thing.
Otherwise, even in a small room, smallest woofer I would even consider for 2 speakers only, cannot have a woofer smaller than 6.5", and preferably 8". I mostly listen to jazz these days. The subwoofer, provided it can translate also the more melodic, upper sub bass notes as well as the lower frequencies, really fits well in jazz. From the standards that utilize the double bass, to the modern fusion version of bass effects as well. 5" or less woofers that do mid 40s hz and above, will need a sub, IMO.
