No mention of size restriction for bikes on the website:

Accepted travel cases:

Recognised bicycle bag.

We will accept all single bicycle bags as illustrated, but these may be subject to additional fees.
Equipment accepted:

One non-motorised bicycle.

Preparing bicycles for travel:

* Bicycle pedals must be removed (or fixed inwards).
* Handlebars must be fixed sideways.
* The bicycle must be contained in a protective case or bag.
* You may wish to deflate the tyres to reduce risk of damage.

http://www.britishairways.com/travel...t/public/en_gb