There is no need to go for a different planet size. Just change the rotation rate of the planet. Since the current models suggest that, just after the moon was formed, the length of a day was about 2 hours, a 12-hour day certainly seems reasonable. Plants and animals will need to have evolved the appropriate circadian rhythms, but that's no problem.

There is no obvious gross effect of a shorter day that I can think of, but someone is sure to come up with a suggestion or two.

EDIT
 http://www.artificial-gravity.com/Dissertation/2_2.htm reviews various research efforts on the subject, and establishes 1 rpm as a reasonable lower limit on negative effects. Since a 12-hour day has a rotation rate 720 times slower, there seems no reason to worry.

It's worth pointing out that there are hundreds of rotating restaurants. For instance, the Space Needle restaurant rotates at 1 revolution per hour, and nobody seems to have problems with it. Of course, exposure is relatively brief, an hour or two, so there might conceivably be long-term effects, but this seems unlikely.