Spain's biggest fashion retailers have agreed to re-size their clothing in an attempt to better reflect the real sizes of women and reduce pressure on them to conform to a skinny image.

Health Minister Elena Salgado announced on Tuesday that the National Consumer Institute will measure 8,500 women between 12 and 70 years of age with the aim of "defining standard patterns and redefining and normalising sizes," according to an agreement between fashion houses and the ministry.

Giants such as Inditex, which owns fast-fashion retailer Zara, El Corte Ingles, Mango and Cortefiel are behind the agreement, aimed at making fashion sizes "truthful, homogenous and comprehensible".

Mannequins in shop windows of the companies signed up will now be at least a European size 38, while size 46 will no longer be considered outsize.

The government hopes that by selling bigger, more realistic sizes, it will discourage women from trying to conform to a stick-thin ideal as presented on fashion catwalks.

In an attempt to help curb weight-related disorders such as anorexia, Spain last year barred models below a certain weight from Madrid catwalks while similar measures were taken in Italy.

"If everything goes well as we expect then of course we will continue with (a resizing) for men - but it seems that men's sizes are more advanced," Ms Salgado told reporters.

Once the study is completed, retail sizes will be unified over an 18-month period.

Labels will include not just the overall size but specifics such as hip and shoulder size.