Look for a pair of RA Labs center channel speakers, a sealed box design. They work great on a book shelf. They are 7 inches high, 21 inches wide (tweeter/2 mid range). The often come up used for about 250 for the pair. RA was one of Roy Allison's design efforts that worked well in a number of sizes. There is a passive sub that works well with all of the RA speakers.
Budget bookshelf speakers FOR a bookshelf...
am looking for a set (under $600) of bookshelf speakers that will actually sound good in a bookshelf. not on top, but on a shelf with a shelf of books above and below them. max of 10" wide by 14" high. i like all kinds of music, from abba to metallica, the planets to rammstein, al dimeola to joe bonamassa. used of course is just fine :)