No, but they must be big enough to reach the footpegs.
Pillion Law
Without going through all the road traffic acts, here's a brief synopsis:
1) You must hold a full licence for the machine you're riding, wear a safety helmet etc.
2) Your bike must be adapted to carry a pillion (a rear seat and passenger footrests).
3) Your pillion must wear a safety helmet and sit astride the machine facing forward. There's no legal age required for riding pillion but he/she must big enough to hold on, sat astride with feet on the foot pegs (so there is a size limit) and able to fully understand the instructions you give.
http://www.rmif.co.uk/article.aspx?id=162