The length of the fixed term to give is really whatever you feel comfortable with.
When the Housing Act 1988 came into force, it used to be the case that a minimum term of six months was required under section 20 Housing Act 1988. However, section 96 Housing Act 1996 abolished this rule for all new tenancies from 27 February 1997 by inserting a new section 19A into the Housing Act 1988. This new rule even meant that a fixed term was no longer necessary and in fact a periodic tenancy can be granted from the outset.
The new section 19A which applies to all tenancies from 27 February 1997 provides:
19A Assured shorthold tenancies: post-Housing Act 1996 tenancies.
An assured tenancy which—
(a )is entered into on or after the day on which section 96 of the Housing Act 1996 comes into force (otherwise than pursuant to a contract made before that day), or
(b) comes into being by virtue of section 5 above on the coming to an end of an assured tenancy within paragraph (a) above,
is an assured shorthold tenancy unless it falls within any paragraph in Schedule 2A to this Act.
As a result, we used to recommend a blanket 3 month fixed term (except for students) but changes to council tax rules from 1 April 2013 means the position is more complex now. Nowadays, we recommend a fixed term of at least 6 months but the only reason for recommending 6 months over 3 months is for council tax purposes. The problem with a 3 month tenancy since the council tax changes is if a tenant were to abandon, the landlord could be liable to pay council tax for the period of abandonment despite the tenant not giving notice. However, the same situation on a 6 month tenancy would usually mean the tenant would remain liable for council tax until proper notice was given.
See also Ratcliffe v Parkes – High Court appeal which confirms that a fixed term can be for any period including less than 6 months.