To reproduce: 1. Start konsole. 2. Select "New | Root Console". 2. Stretch vertically with mouse. 4. Switch between "Terminal No. 1" and "Root Console No. 2" repeatedly. The window will shrink until its only about 6 lines tall. It should stay the size it was expanded to.
This appears to be the same as this KDE bug: http://bugs.kde.org/db/64/6431.html
Fixed.