At present I would argue there is no such provision in the Equality Act 2010 for protection against discrimination on the grounds of size. It is not widely perceived as a disability giving rise to protection or obliging employers to make reasonable adjustments to accommodate their larger work force.
A test case is currently before the European Court of Justice to determine whether EU law (which by implication would include UK law) provides protection for the obese. The case was reported in the Times yesterday and is being brought by a child minder who is 25 stone and employment lawyers are awaiting the outcome with interest.
If obesity is covered under the Equality Act and requires protection this would be a growing concern as statistics show that the number of obese adults in the UK is rising. The Times statistics show that currently 64% of adults in Britain are obese which has more than doubled n the last three decades. If covered the protection would not just extend to discrimination but may also cover the requirement on employers to make reasonable adjustments to cover obesity. This could include a wide range of measures such as special seating, car parking spaces closer to the office, lifts etc.
As always watch this space for the update on whether the reference to the ECJ is successful.