"To define the shape" means to give a clear and specific description or outline of something so that its form or structure is easily understood. Full definition
The belt helps define the shape of the skirt and makes it a tad more formal for a work environment. (moretomrse.com)
In profile, the bigger hood give the cars a more defined shape as compared to its predecessor while at the rear there are new tail lamps to add to the looks. (carwale.com)
The week ahead could help define the shape of the next election. (nextleft.org)