It really depends on the kind of sound effect you are trying to create. As a general rule, longer impulses are better for creating ambiences and soundscapes but, having said that, if you take a very short sound and convolve it with a long impulse you might get an interesting spot effect. It all depends on the spectra and length of the two sounds you're convolving and what you're hoping to get. I've found that convolution using exotic impulses is very good for producing ghostly, breathlike effects, stingers and dark ambiences.
The "Pulses" folder in Virtuasonic's impulse library called Synesthesis comprises very short impulses that produce interesting effects. Stacking impulses can also produce interesting results, particularly short following long or vice versa.
The whole process of unconventional convolution is somewhat alchemical and the results can be unpredictable unless you've got a good sense of pitch and what musical notes equate to on a spectrograph. Some rules of thumb are:
the longer the impulse you're using, the greater the smear or blur that will be imparted to your sound.
Prominent frequency components in either the impulse or the audio you wish to treat will be preserved, so if you have a very musical impulse, your audio will most likely take on those qualities. If you are treating a piece of music then long notes in particular will be preserved in some form.
Frequency sweeps used as impulses will produce frequency dependent delays. Sound effects used as impulses faded in or faded out will produce reverse reverb qualities and reverb qualities respectively.
Here's an excelent article from Electronic Musician that goes into a lot of detail on how to get the most out of convolution.