There is no instrument smaller or higher pitched than the piccolo trombone. The next closest trombones in size and range are the E-flat piccolino trombone and the B-flat soprano trombone. The soprano trombone is pitched an octave below the piccolo trombone, and is in the same range as a B-flat trumpet. In fact, it is common to find the soprano trombone referred to as a ‘slide trumpet’.

The piccolino trombone is pitched a perfect fourth above the soprano trombone and a perfect fifth lower than the piccolo trombone. The smaller the instrument, the higher its range and the shorter the slide. As far as I’ve been able to determine, the piccolino trombone is not currently made or sold and is pretty much the unicorn of the trombone family. The piccolino trombone was sometimes used in Moravian trombone choirs. (To see/hear a piccolino trombone, check out this video at 2:38.)